# Introduction to missingHE

The package `missingHE` is specifically designed to facilitate the implementation of different types of missing data models for trial-based health economic evaluations under a Bayesian statistical framework. Justification for the use of Bayesian inference in economic evaluations is related to the decision-making nature of the research question, which requires to assess the impact of different sources of uncertainty both on the statistical and cost-effectiveness conclusions.

Although frequentist methods are popular among practitioners, they require some ad-hoc steps for properly quantifying the uncertainty around the decision problem based on key cost-effectiveness estimates. Examples include the use of some form of bootstrapping approach to generate standard cost-effectiveness outputs (e.g. cost-effectiveness planes and acceptability curves), or a multiple imputation approach for handling missingness uncertainty. Even though these steps can lead to statistically valid results, in practice, when the complexity of the model is increased (for example to deal with common statistical issues of the data such as correlation, clustering or skewenss), the task of correctly quantifying uncertainty around estimates of interest may become incredibly challenging. The Bayesian approach, through standard Markov Chain Monte Carlo (MCMC) algorithms, can overcome these issues as it provides a more flexible approach to fit models of increasing complexity with relatively limited implementation difficulties. In addition, posterior distributions for any model quantity can be obtained while simultaneously handling multiple types of data idiosyncrasies (including missing data), and correctly propagating and quantifying the uncertainty on each unobserved quantity in the model.

Different types of missing data approaches exist in the literature, each with its own advantages and disadvantages according to the specific assumption made about the missing values. `missingHE` implements three types of missingness models: selection, pattern mixture, and hurdle models. All models are implemented using the `BUGS` language and the freely-available Bayesian program `JAGS`. For technical details and an overview of the software see https://mcmc-jags.sourceforge.io/.

For each model, the package provides a series of ancillary functions for assessing convergence of the MCMC algorithm, checking the fit to the observed data, extracting and summarising posterior estimates about model parameters and cost-effectiveness results. This brief tutorial aims at helping getting started with the package and its main functions. Throughout the document, the built-in data set called `MenSS` is used as a toy example, which becomes directly available when installing and loading `missingHE` in your `R` workspace. It is important to remember that `missingHE` is designed to handle missing values only in the effectiveness and cost outcome variables (no missing values in the covariates are allowed).

If you would like to have more information on the package, or would like to point out potential issues in the current version, feel free to contact the maintainer at a.gabrio@maastrichtuniversity.nl. Suggestions on how to improve the package are also very welcome. A development version of the package is also available from the author’s main GitHub repository at https://github.com/AnGabrio/missingHE.

## 1 Modelling Framework

The modelling framework of `missingHE` can be differently specified by the user according to three aspects of the analysis: 1) choice of the parameterisation structure of the model; 2) choice of the missingness approach; 3) format of the dataset. In this introductory document, key elements of the modelling framework and the available parameterisation structures are provided. To ease presentation, these concepts are here only introduced with respect to the analysis of a cross-sectional dataset, which has historically represented the default analysis setting in the health economics literature. More advanced models and customisation options available in `missingHE` are discussed in separate vignettes, including non-ignorable models, model customisation, and longitudinal data analysis.

Trial-based health economic data usually consist in the cross-sectional cost-effectiveness pair of variables \((e_{it},c_{it})\), collected on the \(i\)-th patient assigned to the \(t\)-th treatment in the study, for \(i=1,\ldots,N\) patients and \(t=1,\ldots,T\) treatment options. In many jurisdictions the recommended effectiveness variable \(e_{it}\) is a Quality Adjusted Life Years (QALYs) measure, which is typically obtained by aggregating utility scores computed from the patients’ answers to generic health-related quality of life questionnaires (e.g. EQ-5D) collected at given time intervals in the study. The cost variable \(c_{it}\) denotes the sum of all relevant patient-level costs (e.g. direct/indirect medical costs, productivity losses, etc.), which are often collected from the patients’ answers to different types of healthcare-resource utilisation questionnaires or medical records in the study.

In `missingHE`, the observed variables \((e_{it},c_{it})\) are modelled using a joint bivariate probability distribution with density \(p(e_{it},c_{it}\mid \boldsymbol \theta_t)\), as a function of a vector of relevant parameters \(\boldsymbol \theta_t\), and implemented using the following conditional factorisation

\[\begin{equation} p(e_{it},c_{it} \mid \boldsymbol \theta_t) \quad = \quad p(e_{it} \mid \boldsymbol \theta_{et})p(c_{it} \mid e_{it}, \boldsymbol \theta_{ct}), \tag{1} \end{equation}\]

where \(p(e_{it}\mid \boldsymbol \theta_{et})\) denotes the marginal effectiveness distribution and \(p(c_{it} \mid e_{it}, \boldsymbol \theta_{ct})\) the conditional cost distribution given \(e_{it}\), with \(\theta_{et}\) and \(\theta_{ct}\) being the treatment-specific set of parameters indexing the marginal effectiveness and conditional cost distributions, respectively. To simplify notation, we now drop the treatment index \(t\) from model parameters.

Both outcome densities in Equation (1) are specified using some parametric forms for the probability distributions to describe the underlying uncertainty in the cost-effectiveness data. The set of model parameters can be generally represented as \(\boldsymbol \theta=(\boldsymbol \phi(x),\boldsymbol \psi(x))\), where: \(\boldsymbol \phi(x)=(\phi_e(x),\phi_c(x))\) denotes the set of location parameters (e.g. mean) and \(\boldsymbol \psi(x)=(\psi_e(x),\psi_c(x))\) the set of ancillary parameters (e.g. variance) indexing the outcome distributions, both depending on some vector of potential baseline covariates \(x\) (e.g. age, sex, etc…). While it is possible for both \(\boldsymbol \phi\) and \(\boldsymbol \psi\) to explicitly depend on \(x\), the model specification is usually simplified assuming that these only affect the location parameters through a generalised linear formulation. For example, the location parameter of the effectiveness model can be specified as:

\[\begin{equation} g_e(\phi_{ie}) \quad = \quad \alpha_{0} + \sum_{k=1}^K \alpha_kx_{ik}\,\,[+ \ldots], \tag{2} \end{equation}\]

and the location parameter of the cost model as:

\[\begin{equation} g_c(\phi_{ic}) \quad = \quad \beta_{0} + \sum_{k=1}^K \beta_kx_{ik} + \beta_f e_{i}, \,\,[+ \ldots]. \tag{3} \end{equation}\]

where the regression coefficient \(\beta_f\) is included to capture the possible dependence between \(c_{i}\) and \(e_{i}\) at the location level. Covariates may be included into the model at different scales using the link functions \(g_e(\cdot)\) and \(g_c(\cdot)\), being either identity, logarithmic or logit, depending on the type of outcome and distribution specified. Note that `missingHE` expands any categorical covariates to a set of dummy variables. In addition, Equation (2) and Equation (3) can be extended, as indicated by the term \([+ \ldots]\), to include additional terms into the model, e.g. clustering effects to account for the possible multilevel structure of the data. The objective of the statistical analysis is the estimation of population average effectiveness and cost parameters \(\boldsymbol \mu=(\mu_{e},\mu_{c})\) in each treatment group and to quantify the (posterior) uncertainty around them. Estimates of these parameters can be obtained from Equation (2) and Equation (3) either from their respective regression parameters after mean centring all covariates or through repeatedly sampling from the predictive distribution of the outcomes using Monte Carlo integration.

Assuming that the location parameters are modelled using a linear predictor form as in Equation (2) and Equation (3), priors on the corresponding sets of regression coefficients can be specified as \(\boldsymbol \alpha=(\alpha_0,\ldots,\alpha_K) \overset{\text{iid}}{\sim} \text{Normal}(\mu_{\alpha},\sigma_{\alpha})\) and \(\boldsymbol \beta=(\beta_0,\ldots,\beta_K,\beta_f) \overset{\text{iid}}{\sim} \text{Normal}(\mu_{\beta},\sigma_{\beta})\), where the notation \(\mu_{\cdot}\) and \(\sigma_{\cdot}\) denotes the prior mean and standard deviation, respectively. By default, `missingHE` assumes the “minimally informative” prior values of \(\mu_{\cdot}=0\) and \(\sigma_{\cdot}=1000\) for all regression coefficients. When genuine prior knowledge on specific parameters is available, it is possible to modify the default priors and elicit the corresponding information into the model. As for the ancillary parameters, the choice of the prior values depends on the form of the probability distribution selected according to the modelling choices available in `missingHE`.

## 2 Missingness Approach

`missingHE` focusses exclusively on handling missingness in cost and effectiveness measures (\(o_i=c_i,e_i\)) assuming the set of baseline variables \(\boldsymbol X_i\) to be fully-observed for all individuals. When analysing incomplete outcome data, it is essential to investigate the reasons behind the missingness process, often referred to as the missing data mechanism. Since this is typically not under the control of the investigators, unverifiable assumptions about the missing data mechanism have to be made, and the validity of the analysis depends on whether the assumptions formulated hold for the data at hand. Formally, given a certain outcome variable \(o_i\), its missing data mechanism can be defined as a probability model for the distribution of the missingness indicators \(m_i\), denoting whether the corresponding value is observed \(o_i=o^{\text{obs}}_i (m_i=0)\) or missing \(o_i=o^{\text{mis}}_i (m_i=1)\), conditional on \(o^{\text{obs}}_i\), \(o^{\text{mis}}_i\), and \(\boldsymbol X_i\). Depending on the assumed relationship between \(m_i\), \(o^{\text{obs}}_i\), \(o^{\text{mis}}_i\) (and \(\boldsymbol X_i\)), three broad types of mechanisms can be distinguished according to Rubin’s taxonomy: missing completely at random (MCAR); missing at random (MAR); missing not at random (MNAR).

Under a MCAR mechanism:

\[\begin{equation} p(m_i\mid o^{\text{obs}}_i,o^{\text{mis}}_i,\boldsymbol X_i,\boldsymbol \eta)=p(m_i\mid \boldsymbol \eta), \tag{4} \end{equation}\]

where \(\boldsymbol \eta\) denotes the set of parameters indexing the missingness model. A key consequence of MCAR is that any method that yields valid inferences in the absence of missing values will also be valid when focussing on the observed data alone, including the set of individuals with fully-observed data (“complete cases”).

Under a MAR mechanism:

\[\begin{equation} p(m_i\mid o^{\text{obs}}_i,o^{\text{mis}}_i,\boldsymbol X_i, \boldsymbol \eta)=p(m_i\mid o^{\text{obs}}_i,\boldsymbol X_i, \boldsymbol \eta). \tag{5} \end{equation}\]

A key consequence of MAR is that missing values can be validly “predicted” using the observed data and a correct model for \(o_i\). Both MCAR and MAR are often referred to as ignorable mechanisms in that, once it is established that \(p(m_i\mid o_i,\boldsymbol X_i)\) does not depend on \(o^{\text{mis}}_i\), it is possible to obtain valid inferences given a correctly specification for \(p(o_i\mid \boldsymbol X_i)\).

Finally, under a MNAR mechanism:

\[\begin{equation} p(m_i\mid o_i,\boldsymbol X_i, \boldsymbol \eta)=p(m_i\mid o^{\text{obs}}_i,o^{\text{mis}}_i,\boldsymbol X_i, \boldsymbol \eta). \tag{6} \end{equation}\]

A MNAR mechanism is often referred to as non-ignorable or informative because the model assumed for \(p(m_i\mid o_i,\boldsymbol X_i)\) must be specified and its choice will drive the results of the analysis. Thus, without relying on external information, identification of the model is driven by unverifiable assumptions and conducting sensitivity analysis to assess the robustness of the results to a range of plausible missingness assumptions becomes imperative.

Under MNAR, almost all standard analysis methods are invalid, and a “principled” approach to missingness is instead required to obtain valid inferences. We specifically refer to principled methods for missing data, also referred to as joint models, as those which are based on a well-defined statistical model for the complete (i.e. observed and missing) data and explicit assumptions about the missing data mechanism. Alternative approaches have also been developed that do not require the specification of a joint model but instead identify the model through specific assumptions about the missing values. `missingHE` implements three methods to handle non-ignorable missingness in the context of trial-based cost-effectiveness analysis: two joint modelling approaches, namely selection and pattern mixture models, and one approach which relies on a combination of assumptions about the distribution of the outcome and informative assumptions about the missing values, the hurdle model. A summary presentation of these approaches is now provided.

## 3 Selection Models

The joint distribution of an outcome \(o_{i}\) (\(e_{i} \;\text{or}\; c_{i}\)) and its missingness (\(m_i\)) is specified in terms of a complete data model for the outcome and an explicit model for the probability of missingness conditional on the possibly unobserved data:

\[\begin{equation} p(o_{i}, m_{i} \mid \boldsymbol \theta, \boldsymbol \eta) \quad = \quad p(o_{i} \mid \boldsymbol \theta)p(m_{i} \mid o_{i},\boldsymbol \eta), \tag{7} \end{equation}\]

where \(\boldsymbol \theta\) and \(\boldsymbol \eta\) denote the two (distinct) sets of parameters indexing the outcome and missingness model, respectively. While for \(o_{i}\) the model specification varies depending on the type of outcome and distributional assumptions (see Section 1), for \(m_{i}\) `missingHE` uses a common model structure where the missingness probability \(\pi^m_i\) is modelled as function of other quantities through a logistic regression:

\[\begin{equation} \text{logit}(\pi^m_i)=\gamma_{0}+ \sum_{k=1}^K \gamma_k x_{ik} + \delta o_{i} \,\,[+ \ldots], \tag{8} \end{equation}\]

where \(\gamma_{0}\), \(\boldsymbol \gamma=(\gamma_1,\ldots,\gamma_K)\) and \(\delta\) denote the intercept, the set of covariates-specifc coefficients, and the coefficient capturing the impact of the unobserved values (in \(o_i\)) on the missingness probability, respectively. Additional terms, e.g. clustering effects, may also be included in Equation (8) as indicated by the term \([+ \ldots]\). Due to the missing values in \(o_i\), the model can only be identified by imposing some restrictions. These are implicitly defined from a combination of parametric assumptions about \(p(o_i\mid \boldsymbol \theta)\) and dependence structure assumed for \(p(m_i \mid o_i,\boldsymbol \eta)\). As a result, sensitivity analysis to assess the impact of alternative missingness assumptions on the inferences can be done by: varying the distributional assumption of \(p(o_i\mid \boldsymbol \theta)\); varying the modelling structure in Equation (8); and, within a Bayesian setting, varying the prior distribution on \(\delta\).

## 4 Pattern Mixture Models

The joint distribution of outcome and missingness is specified in terms of a conditional model for the outcome given the missingness indicators and a marginal model for the probability of missingness:

\[\begin{equation} p(o_i, m_i \mid \boldsymbol \theta, \boldsymbol \zeta) \quad = \quad p(o_i \mid m_i, \boldsymbol \nu)p(m_i \mid \boldsymbol \zeta), \tag{9} \end{equation}\]

where \(\boldsymbol \nu\) and \(\boldsymbol \zeta\) denote the two (distinct) sets of parameters indexing the conditional model for the outcome and the marginal model of missingness, respectively. In contrast to the missingness model in Section 3, the marginal model for \(m_i\) corresponds to a model for the different missingness patterns \(d^m_i\) to which individuals can be assigned (according to the values of the indicators \(m_i\)). Following standard practice in the literature, to ease specification of \(p(m_i \mid \boldsymbol \zeta)\), `missingHE` directly models the missingness pattern indicator \(d^m_i\) using a Multinomial distribution

\[\begin{equation} d^m_i \sim \text{Multinomial}(\boldsymbol \pi^{d^m}), \tag{10} \end{equation}\]

with \(\boldsymbol \pi^{d^m}\) being the set of probabilities for each possible pattern, which is modelled through a joint prior \(\boldsymbol \pi^{d^m} \sim \text{Dirichlet}(\boldsymbol a^d)\), where \(\boldsymbol a^d\) denotes the set of prior concentration parameters. By default, `missingHE` assumes a minimally informative prior on these parameters by setting \(\boldsymbol a^d=\boldsymbol 1\) to assign the same weight to each pattern probability.

Note that, the conditional distribution of the outcomes is not always identifiable because, for all but the complete cases, some values are missing. To overcome this problem, some type of identifying restrictions are imposed on the model to ensure that all pattern-specific outcome distributions can be identified. Typically, these restrictions are constructed using information from other identifiable patterns, and therefore imply some form of ignorability of the missingness mechanism. Different types of identifying restrictions exist and `missingHE` provides the user with two choices: complete case (CC) and available case (AC) restrictions. For example, under the CC restrictions, `missingHE` identifies the parameters of any unidentified outcome distributions in Equation (9) using the corresponding estimates from the complete case pattern.

Exploration of non-ignorable assumptions is achieved by including sensitivity parameters \(\delta\) within the type of restrictions used to identify the model under ignorability. `missingHE` assesses MNAR with respect to the mean parameters \(\mu\) by additively including \(\delta\) to the imposed restrictions on pattern-specific parameters \(\mu^{d^m \star}\) that are not identifiable from the data. For example, using the CC restrictions, `missingHE` identifies the marginal mean of a given outcome (\(e\) or \(c\)) in the fully-missing pattern as:

\[\begin{equation} \mu^{(1,1)} = \mu^{(0,0)} + \delta \tag{11} \end{equation}\]

where \(\mu^{(1,1)}\) and \(\mu^{(0,0)}\) denote the marginal mean in the fully-missing and complete case pattern, respectively. Sensitivity analysis can then be conducted by: varying the type of identifying restrictions imposed; varying the prior distribution on \(\delta\).

## 5 Hurdle Models

Although hurdle models do not explicitly specify the joint distribution \(p(o_i,m_i)\), and therefore cannot be classified as principled missingness methods, they can be tailored to assess the impact on the results of alternative non-ignorable missingness assumptions. A key advantage of hurdle models in trial-based economic evaluations is their ability to explicitly handle the occurrence of many identical values in the outcomes, also referred to as “structural values” (e.g. zero costs or perfect health state), which often make model implementation quite problematic.

For a given outcome variable \(o_i\) (i.e. \(e_i\) or \(c_i\)), `missingHE` implements an hurdle model to handle a given structural value \(s\) by creating a corresponding indicator variable \(d^s_i\), which takes value \(1\) when \(o_i=s\) and \(0\) otherwise. The probability of observing a structural value \(\pi^s_i\) is then estimated via logistic regression

\[\begin{equation} \text{logit}(\pi^s_i)=\gamma_{0} + \sum_{k=1}^K \gamma_k x_{ik} \,\,[+ \ldots] \tag{12} \end{equation}\]

as function of the intercept \(\gamma_{0}\), the covariates-specific coefficients \(\boldsymbol \gamma=(\gamma_1,\ldots,\gamma_K)\), and other (possibly) relevant terms \([+ \ldots]\) (e.g. clustering effects). Estimates for the marginal probability of being associated with a structural value \(\pi^s\) are retrieved by either applying the inverse logit function to Equation (12) after mean-centring all covariates or by repeatedly sampling from the posterior predictive distribution of the outcome variables through Monte Carlo Integration. Finally, estimates for the marginal mean of \(o_i\) are obtained from the linear combination

\[\begin{equation} \mu=(1-\pi^s)\mu^{(d^s=0)}+\pi^s s, \tag{13} \end{equation}\]

where \((1-\pi^s)\) and \(\mu^{(d^s=0)}\) denote the marginal probability of not being associated with the structural value \(s\) and the marginal mean for the non-structural component of the outcome, respectively. `missingHE` retrieves estimates of \(\mu^{(d^s=0)}\) using the same modelling framework and distributions as in Section 1, but applied to \(o_i\) for only the non-structural cases, i.e. all individuals for whom \(d^s_i=0\). Making a parallel with the missing data literature, depending on whether no or some covariates are included in Equation (12), the underlying mechanism has been named structural completely at random (SCAR) or structural at random (SAR). However, a key difference with respect to MCAR or MAR is that failure to correctly specify Equation (12) will always lead to invalid inferences since estimates of \(\pi^s\) are needed in Equation (13).

When some outcome data are missing, values of \(d^s_i\) are unknown for at least some individuals and, under MAR, valid inferences can be obtained provided a correct specification of Equation (12) and of the model fitted to the non-structural values. However, hurdle models allow to explore the robustness of the results to specific non-ignorable assumptions, therefore allowing to perform a simple type of sensitivity analysis. This is achieved by arbitrarily set \(d^s_i\) to assign the individuals with missing outcome values to either the structural (\(d^s_i=1\)) or non-structural (\(d^s_i=0\)) component of the mixture, and explore alternative configurations in terms of the proportions of these values (e.g. between treatment groups).

## 6 The MenSS trial

In the following, we use data from a real clinical study as a running example to familiarise the user with the key features of `missingHE`. The MenSS trial was a pilot randomised trial whose participants were young men at risk of sexually trasmitted infections (STIs) attending sexual health clinics in England. The trial aimed at evaluating the feasibility of extending the design to a full-scale trial and to provide a preliminary assessment of the cost-effectiveness of two alternative interventions: standard of care (SoC) versus the Men’s Safer Sex website (MenSS), consisting in a web-based interactive intervention aimed at giving advice about sexual health and STIs.

After loading the package `missingHE`, for example by typing `library(missingHE)`, the MenSS dataset (`MenSS`) is directly imported into the `R` workspace and can be inspected using the command

    > rbind(head(MenSS), tail(MenSS))

to show the first and last few rows:

    #>   id   u.0     e  c age ethnicity employment sex_inst.0 sex_inst sti.0 sti site   trt
    #>    1 0.725    NA NA  23         0          0         25       NA     0   1    1   SoC
    #>    2 0.848 0.924  0  23         0          1         30       50     0   0    3   SoC
    #>    3 0.848    NA NA  27         1          0          6        3     0   0    3   SoC
    #>    4 1.000    NA NA  27         0          0         20        0     0   0    1   SoC
    #>    5 0.796    NA NA  18         0          0          1       99     0   0    3   SoC
    #>    6 1.000 0.943  0  25         1          1         20       NA     1   0    1   SoC
    #>  154 1.000    NA NA  27         1          1          2       10     0   0    2 MenSS
    #>  155 0.882    NA NA  18         1          1          3       NA     0   0    3 MenSS
    #>  156 1.000    NA NA  21         1          1         12       40     0   0    3 MenSS
    #>  157 1.000    NA NA  21         0          0          8        0     0   0    3 MenSS
    #>  158 0.725    NA NA  28         1          1          0       NA     0   0    3 MenSS
    #>  159 1.000    NA NA  38         0          0          6       NA     0   0    3 MenSS

The dataset consists of \(159\) individuals grouped in two arms, where `trt`\(=1\) indicates the SoC and `trt`\(=2\) indicates the MenSS intervention, with `id` denoting the participant identification number.

Key health economic variables are `u.0`, `e` and `c`, which denote the baseline utility score, QALY and Total cost outcomes, respectively. All baseline variables are fully-observed, whereas all outcome variables are partially-observed in both treatment arms. A total of 46 (29%) individuals has observed QALYs and Total costs, of which 0 in the SoC and 0 in the MenSS arm, while the remaining 113 individuals have both `e` and `c` missing. Figure 1 and Figure 2 show histograms of the observed distributions for the QALY and Total cost variables in `MenSS`, separately by treatment arm. We note that, among the observed cases, 17 (37%) individuals are associated with structural values of perfect health (\(s^e=1\)), while 12 (26%) individuals have structural values of no service use (\(s^c = 0\)).

Figure 1: Histograms of the observed data distributions for the QALYs in the SoC and MenSS arm.

Figure 2: Histograms of the observed data distributions for the Total costs in the SoC and MenSS arm.

## 7 Model fitting

`missingHE` relies on `JAGS` through the dedicated functions `selection`, `pattern`, and `hurdle` to implement the desired missingness approach while providing a series of customisation options. Although some of these options are specific to a given approach, many of the available arguments are shared across the three functions to ensure consistency and uniformity when implementing the model in Section 1. In particular, key options include: choice of the probability distribution for the effectiveness and cost variables, choice of the regression model for each outcome, and type of missingness mechanism assumed.

A set of pre-defined choices in terms of probability distributions, with built-in parameterisations, for the effectiveness (`e`) and/or cost (`c`) variables are available. The user may select a given distributional choice by passing the corresponding `missingHE` name to the arguments `dist_e` and `dist_c` of the desired function. The outcome model \(p(e,c)\) requires a separate specification of the effectiveness and cost model formula following Equation (1) using the notation:

    model.eff <- e ~ trt + ...
    model.cost <- c ~ trt + ...

where `e`, `c` and `trt` indicate the effectiveness, cost and treatment variable names stored in the data set, while `...` is a suitable form for the combination of covariates to be used in each model. Dependence between the two models can be specified by adding the term `e` as an additional covariate in the cost model. Note that the treatment indicator `trt` must be included into both regression formulae and should be coded as a factor variable in the data set. The type of missingness assumption for each outcome is specified through the argument `type`, by setting `type = "MAR"` or `type = "MNAR"` to select between a MAR or MNAR mechanism, respectively.

To provide a concise overview of key generic options in `missingHE`, let us denote with `fit.function` the name of one function among `selection`, `pattern` or `hurdle`. Then, a suitable call to `fit.function` is the following.

    fit.model <- fit.function(data = data, model.eff = model.eff,
      model.cost = model.cost, dist_e = dist_e, dist_c = dist_c, type = type, ...)

where `data` is a dataframe containing the data, `model.eff` and `model.cost` are the formulae for the effectiveness and cost regression models, `dist_e`, `dist_c` and `type` are character values indicating the chosen outcome distributions and type of missingness mechanism, while `...` indicates additional optional arguments. In practice, unless there is a clear understanding of the implications of any change to the default model structure, the user is not required to modify the default values of these optional arguments. The only exception is related to: number of MCMC chains (`n.chains`), number of iterations (`n.iter`) and values of the parameters of the prior distributions (`prior`). As an example, consider the default priors for the linear predictor coefficients in the effectiveness and cost model, namely \(\boldsymbol \beta \overset{\text{iid}}{\sim}\text{Normal}(\mu_{\beta},\sigma_{\beta})\) and \(\boldsymbol \alpha \overset{\text{iid}}{\sim}\text{Normal}(\mu_{\alpha},\sigma_{\alpha})\). Suppose the user wants to select different prior mean and standard deviation values for these parameters. This can be done by typing

    myprior <- list("beta.prior" = c("norm", 0, 0.01), "alpha.prior" = c("norm", 0, 0.01))

which instructs `missingHE` to separately assign normal distributions to each regression coefficient in the models with prior mean of \(0\) and precision (inverse of variance) of \(0.01\). The list `myprior` can be then passed to the optional `prior` argument in the chosen model fitting function to implement the model using the updated priors.

    fit.model <- fit.function(data = data, model.eff = model.eff,
      model.cost = model.cost, dist_e = dist_e, dist_c = dist_c,
      type = type, prior = myprior)

Note that the list and its elements containing the custom prior specification need to be named using the `missingHE` accepted terminology for parameter and distribution names. If the option `save_model` is set to `TRUE`, the `JAGS` model template is saved in the current working directory and can be used to further modify the model structure with a higher degree of flexibility. This, however, will require a new compilation of the model and, at present, the new model has to be run using dedicated commands, such as `rjags` from the `R2jags` package.

## 8 Fitting models under MAR

To ease presentation, we consider the following common model specification for \(p(e_i)p(c_i\mid e_i)\): `e` and `c` are both normally distributed; `u.0` is included as a covariate in the effectiveness model; a MAR mechanism is assumed for both outcomes. The model is first implemented using a selection approach through the function `selection` under a MAR assumption for `e` (conditional on `u.0`) and `c` using the following command.

The arguments `model.me` and `model.mc` are specific to `selection` and denote the missingness mechanism as in Equation (8) for both outcomes. Since `u.0` is included in `model.me` and there are no covariates in `model.mc`, the QALY and Total cost mechanisms are assumed MAR (`type="MAR"`) conditional on the observed baseline utility and outcome values. The argument `ref = 2` is used to specify which study arm should be used as the reference group in the comparison, i.e. incremental results are reported here as `trt=2` (`"MenSS"`) vs `trt=1` (`"SoC"`).

Next, we specify the same outcome model under a pattern mixture approach, but assuming a logistic distribution for `e`. The model is implemented through the function `pattern`, imposing CC restrictions to identify the model under MAR, using the following command.

The argument `restriction` is specific to `pattern` and denotes the type of restrictions imposed to identify the model. Since `restriction = "CC"` (default), the parameters of the unidentified patterns are identified only based on corresponding estimates from the complete cases under a MAR assumption (`type="MAR"`).

Finally, the model is specified under a hurdle approach but assuming a Beta and Gamma distribution for `e` and `c`, respectively. In particular, individuals with \(e_i=1\) and \(c_i=0\) are assigned to a structural component in the QALY and Total cost model, respectively. The model is implemented through the function `hurdle`, under a SCAR assumption about the structural value mechanism, using the following command.

The arguments `se`, `sc`, `model.se` and `model.sc` are specific to `hurdle` and denote the effectiveness and cost structural values and their corresponding mechanism specification as in Equation (12). Since no covariates are included in `model.se` and `model.sc`, the probability of being associated with a structural value in both outcomes is assumed to not depend on any other variable (`type = "SCAR"`). The model is implicitly identified under MAR since no informative assumption is made about the unobserved outcome data.

## 9 Model assessment

Prior to inspecting the posterior results, it is important to assess model performance and convergence of the MCMC algorithm. `missingHE` provides three functions to compute and implement different types of popular Bayesian model assessment measures and plots:

1. `diagnostic` provides a suit of diagnostic tools to visualise the posterior distribution and assess MCMC convergence for each parameter in the model.

2. `ppc` provides a suit of graphical representations of posterior replications generated from the model, also known as posterior predictive checks, to assess the model fit to the observed data.

3. `pic` provides three alternative predictive accuracy measures, also known as predictive information criteria, which are computed based on the log-likelihood evaluated at the posterior simulations of the model parameters.

Each of these functions accepts as main argument an `R` object of class `"missingHE"` containing the output of a Bayesian model generated using any of the three model fitting functions from Section 8.

### 9.1 Convergence diagnostics

Graphical diagnostics are generated through the function `diagnostic`, whereas numeric measures are displayed together with summary parameter estimates using the `print` function - more on this in Section 10.1.

The function `diagnostic` provides a range of standard diagnostic tools and plots for assessing convergence based on the posterior distribution of a Bayesian model fitted in `missingHE`. Consider the output of the selection model fitted in Section 8 and stored in the object `sm1_mar`; we may use the command

    > diagnostic(x = sm1_mar, type = "traceplot", param = "sd.e")

to produce “traceplots” for the standard deviation effectiveness parameter estimates from the model. In this case, as displayed in Figure 3, no major issues in convergence are identified for the parameters monitored, since the traceplots show acceptable mixing in the two chains.

    #> Loading required namespace: ggmcmc

Figure 3: Checking convergence using the diagnostic function for a model fitted in missingHE, for example through inspection of the traceplots.

With the exception of the argument `x`, which must be an object of class `"missingHE"`, all other arguments can be used to customise the output of the function. A full description of the choices for each argument, including the available types of diagnostic tools and families of parameters, is provided in the help file of the package. A quick description of the available options is the following.

- `type`: the class of diagnostic plot, which must be named according to the options and terminology accepted by `missingHE`. Examples include: density plots (default\(=\)`"denplot"`), traceplots (`"traceplot"`) and autocorrelation plots (`"acf"`).

- `param`: family of parameters to be displayed, named according to the options and terminology accepted by `missingHE`. Examples include: effectiveness location regression coefficients \(\boldsymbol \alpha\) (`"alpha"`) and marginal means \(\mu_e\) (`"mu.e"`) as well as their corresponding cost parameters \(\boldsymbol \beta\) (`"beta"`) and \(\mu_c\) (`"mu.c"`). Default value is `"all"`, indicating all parameters stored in the object passed to `x`. Note that some family of parameters are only available for specific types of model structures and missingness assumptions.

Note that most of the output produced by `diagnostic` is effectively a `ggplot2` object, and can therefore be manipulated by the user using functions from packages which allow to customise such objects.

### 9.2 Posterior predictive checks

The function `ppc` provides a range of graphical posterior predictive checks for assessing the fit of a Bayesian model implemented in `missingHE`. Consider the output of the selection model fitted in Section 8 and stored in the object `sm1_mar`; we may use the command

    > ppc(x = sm1_mar, type = "dens_overlay", outcome = "effects", ndisplay = 20)

to plot the densities of `ndisplay`\(=20\) replications of the effectiveness data (light blue lines) with respect to the density of the observed data (dark blue line), separately in the Soc (`trt=1`) and MenSS (`trt=2`) arm. In this case, as displayed in Figure 4, some discrepancies between the replicated and observed data densities are apparent. This suggests a relatively poor fit of the current model (i.e. the normality assumption of `e`) to the observed data in both arms.

Figure 4: Posterior predictive graphical checks using the ppc function for a model fitted in missingHE, for example through inspection of the overlayed densities.

A full description of the choices for each argument in `ppc`, including the available types of posterior predictive checks, is provided in the package help file. A quick description of the available options is the following.

- `type`: class of posterior predictive check, which must be named according to the available options and terminology accepted by `missingHE`. Examples include: histograms (default\(=\)`"histogram"`), overlayed densities (`"dens_overlay"`) and boxplots (`"boxplot"`).

- `outcome`: outcome variable, named according to the available options and terminology accepted by `missingHE`. Available choices are: effectiveness (`"effects"`), costs (`"costs"`) or both (default\(=\)`"both"`).

- `ndisplay`: number of model-based replications of the data to be displayed for each selected type of outcome and graph (default\(=15\)).

### 9.3 Predictive information criteria

The function `pic` allows to assess the relative predictive accuracy of models fitted in `missingHE` by computing three alternative Bayesian information criteria: Deviance Information Criterion or DIC; Widely Applicable Information Criterion or WAIC; and Leave-One-Out Information Criteria or LOOIC. As an example, consider again the model results stored in the object `sm1_mar`; we may use the command

    > pic_sm1_mar <- pic(x = sm1_mar, criterion = "waic", cases = "cc")

to return a named list containing a series of components used in the calculation of the selected information criterion. In this case, WAIC was selected as specified in `criterion = "waic"`, and its value may be inspected by typing:

    > pic_sm1_mar$waic
    #> [1] 778.837

Note that all information criteria in `missingHE` can only be used as relative measures of fit to compare nested models, with lower values typically indicating better fit. A quick description of the two main arguments in `pic` and some key options is the following.

- `criterion`: type of information criteria: DIC (default\(=\)`"dic"`), WAIC (`"waic"`) and LOOIC (`"looic"`). Alongside the value of the criterion, additional quantities used in its computation are also stored and may be returned.

- `cases`: the group of observed outcome cases for which information criterion should be computed. Available choices include: complete cases (`"cc"`), available observed cases for only the effectiveness (`"ac_e"`) or costs (`"ac_c"`), and all cases (`"all"`).

A full description of the different types of output associated and input choices is provided in the package help file.

## 10 Results inspection

Executing any of the three model fitting functions in Section 8 creates an `R` object of class `"missingHE"`, in which results based on the type of model selected and dataset used are stored for inspection. For instance, consider the object `sm1_mar`, which is a list containing the results of a selection model fitted under MAR for both outcomes. The `R` command

    > names(sm1_mar)

displays the names of the different elements in the list.

    #> [1] "data_set"     "model_output" "cea"          "type"         "data_format"

The objects `data_set`, `model_output` and `cea` are themselves lists, whereas `type` and `data_format` are strings. These elements contain the following output:

- `data_set`: a list storing the raw data and information about the model, such as the effectiveness and cost variables, the total number of individuals, the number of observed and missing outcome values, and the covariate data (if included in the model).

- `model_output`: a list storing posterior samples for key parameters of interest (e.g. mean effectiveness and costs), imputed outcome values, and information about model structure (e.g. distributions and missingness assumption).

- `cea`: a list storing standardised health economics measures, computed based on posterior parameter estimates through the function `bcea` from the package `BCEA`. These results can also be further analysed using tailored functions from `BCEA` to obtain standard cost-effectiveness output (see Section 11.1).

- `type`: a string describing the missingness assumption used to fit the model, i.e. either `"MAR"` or `"MNAR"`.

- `data_format`: a string describing the type of data format used to fit the model, i.e. either `"wide"` (cross-sectional) or `"long"` (longitudinal).

Summary statistical output from `JAGS` can be accessed through the standard `R` notation `sm1_mar$model_output[[2]]` by position (i.e. using “double square brackets”) or alternatively by using the name `model` and `$`, and can be inspected by typing

    > names(sm1_mar$model_output$model)

which produces

    #> [1] "model"              "BUGSoutput"         "parameters.to.save" "model.file"
    #> [5] "n.iter"             "DIC"

The quantities included in the `model` list are those stored in the standard `JAGS` output obtained from `R2jags`. Users who are familiar with `JAGS` and `R` programming may access this output to post-process and further customise the model results.

### 10.1 Summarise parameter estimates

Key quantities of interest are the posterior estimates of the mean effectiveness and cost parameters in each treatment arm \(\boldsymbol=(\mu_{et},\mu_{ct})\). Summary statistics computed based on posterior samples of these parameters may be accessed using the `print` function by typing

    > print(sm1_mar)

which returns the following output

    #>                    mean           sd          2.5%          50%       97.5% Rhat n.eff
    #> alpha[1]      0.5368930   0.07789059     0.3838331    0.5377226   0.6775331 1.05    36
    #> alpha[2]      0.0327397   0.02788612    -0.0186368    0.0330192   0.0893773 1.01   280
    #> alpha[3]      0.4000723   0.08300922     0.2500224    0.3992030   0.5580946 1.06    33
    #> beta[1]     202.4499930  42.57713578   120.7669688  202.8161625 285.6549412 1.00   490
    #> beta[2]     -14.6750179  64.29846065  -139.4274219  -13.0923187 108.7936362 1.00   550
    #> beta_f     -672.5613910 295.58609578 -1245.0750952 -678.2155593 -64.7709302 1.00  1000
    #> gamma_c       0.9070391   0.17805898     0.5730949    0.9002036   1.2899792 1.00   740
    #> gamma_e[1]    0.9105364   0.90051080    -0.7025694    0.8488656   2.8120626 1.00   630
    #> gamma_e[2]   -0.0002425   1.00190276    -2.1136617    0.0557771   1.8328266 1.00   570
    #> p_c           0.7110215   0.03608779     0.6394770    0.7109913   0.7841436 1.00   730
    #> p_e           0.7116876   0.03628157     0.6378623    0.7126475   0.7822730 1.00  1000
    #> s_c         214.1587084  23.75071360   174.6263430  211.5113566 268.3669170 1.01   180
    #> s_e           0.0919924   0.01035372     0.0749754    0.0911320   0.1137791 1.00   890
    #> tau_c         0.0000226   0.00000492     0.0000139    0.0000224   0.0000328 1.01   180
    #> tau_e       122.4355005  26.15895929    77.2460018  120.4088997 177.8945649 1.00   890
    #> tmu_c       194.6971534  34.85214674   126.5430117  193.6629355 265.8209455 1.00  1000
    #> tmu_e         0.9070230   0.01410014     0.8792227    0.9069166   0.9360863 1.00  1000

`missingHE` standardises the format of the output to report summary quantities related to key model parameters, by default the mean effectiveness and costs across treatment arms on their natural scale. Estimates of posterior mean (`mean`), standard deviation (`sd`) and \(95\%\) credible interval boundaries (`2.5%` and `95.5%`) can be used to summarise the posterior distribution of the mean parameters. In addition, the MCMC diagnostics `Rhat` and `n.eff`, corresponding to the potential scale reduction factor and effective sample size numeric diagnostics, are provided and can be used to assess convergence for these parameters.

Summary statistics for the regression coefficients on the scale defined by the linear predictor of the effectiveness and cost models \((\boldsymbol \alpha, \boldsymbol \beta)\), as defined in Section 1, may also be displayed using the `coef` function by typing

    > coef(sm1_mar)

which returns the following output

    #> $Effects
    #>              Mean    SD     QL    QU
    #> (Intercept) 0.537 0.078  0.384 0.678
    #> trtMenSS    0.033 0.028 -0.019 0.089
    #> u.0         0.400 0.083  0.250 0.558
    #>
    #> $Costs
    #>                 Mean      SD        QL      QU
    #> (Intercept)  202.450  42.577   120.767 285.655
    #> trtMenSS     -14.675  64.298  -139.427 108.794
    #> e           -672.561 295.586 -1245.075 -64.771

The regression output is separately reported by outcome. For the above output, the following parameter estimates are reported: the intercept \(\alpha_0=\)`(Intercept)` and baseline utility coefficient \(\alpha_1=\)`u.0` in the effectiveness model; the intercept \(\beta_0=\)`(Intercept)` and effectiveness coefficient \(\beta_f=\)`e` in the cost model.

For each regression parameter, estimates are summarised using posterior means (`mean`), standard deviations (`sd`) and credible interval boundaries (`lower` and `upper`), the latter being calculated assuming by default a credible level of \(95\%\).

### 10.2 Inspect imputed values

The function `plot` allows to visually inspect how missing outcome data have been imputed from a model stored in an object of class `"missingHE"`, using `ggplot2` as the graphical engine. For example, the command

    > plot(sm1_mar, class = "scatter", outcome = "effects")

displays scatter plots of the observed (black dots) and imputed (red dots and lines) effectiveness variables in the `"SoC"` (`trt=1`) and `"MenSS"` (`trt=2`) arm based on the model results stored in `sm1_mar`, as shown in Figure 5. Imputed values are represented in terms of posterior mean values, and their uncertainty by means of corresponding credible intervals. The arguments `class="scatter"` and `outcome="effects"` instruct `missingHE` to display scatter plots for the effectiveness outcome in the MenSS arm.

Figure 5: Scatter plots of observed and imputed effectiveness data in the MenSS arm under normal distributions using a selection model.

Note that, similarly to `diagnostic`, all graphs produced via `plot` can be manipulated using `ggplot2` functions, e.g. to modify the graphics’ aesthetics, text and disposition.

## 11 Summarise health economic results

Results of the economic evaluation stored in an object of class `"missingHE"`, such as `sm1_mar`, can be summarised in a tabular format using the `summary` function by typing

    > summary(sm1_mar)

which returns the output

    #>
    #>  Cost-effectiveness analysis summary
    #>
    #>  CE parameter estimates under MAR assumption
    #>
    #>  Mean effects by intervention
    #>        Mean    SD    QL    QU
    #> SoC   0.889 0.018 0.853 0.923
    #> MenSS 0.923 0.022 0.879 0.967
    #>
    #>
    #>  Mean costs by intervention
    #>       Mean   SD    QL  QU
    #> SoC    215 43.2 133.3 303
    #> MenSS  178 52.7  78.5 279
    #>
    #>
    #>  Mean net monetary benefit by intervention and wtp = 50000
    #>        Mean   SD    QL    QU
    #> SoC   44231  905 42445 45970
    #> MenSS 45971 1101 43787 48167

The results stored in the object `sm1_mar` are reported in terms of posterior summaries, such as mean, standard deviation and \(95%\) credible interval boundaries, for key health economic quantities. These are:

- the mean effectiveness and cost parameters in each arm (\(\mu_{et},\mu_{ct}\))

- the net monetary benefit (NMB) parameter in each arm (\(\mu_{et}\times k - \mu_{ct}\)), calculated at a given willingness to pay (\(k\)) value (default\(=50000\)).

Incremental cost-effectiveness results between the reference arm (`trt`\(=2\)) and each of the other study arms (`trt`\(=1\)) can be obtained by setting the argument `incremental` to `TRUE`.

    > summary(sm1_mar, incremental = TRUE)

The results include:

- the mean incremental effectiveness and cost parameters (\(\Delta_{e}=\mu_{e2}-\mu_{e1},\Delta_{c}=\mu_{c2}-\mu_{c1}\))

- the incremental net monetary benefit (INMB) (\(\Delta_{e}\times k - \Delta_{c}\)), calculated at the given \(k\) value.

- the estimated mean of the Incremental Cost-Effectiveness Ratio (ICER), which quantifies the cost per incremental unit of effectiveness, computed based on posterior samples as \(\text{ICER}=\frac{\Delta_{c}}{\Delta_{e}}\).

The above output compares the cost-effectiveness of the two arms in the `MenSS` data set based on the results of the model stored in the object `sm1_mar`. For example, in this case, incremental results suggest that `"MenSS"` is on average associated with higher QALYs (0.034) and lower Total costs (-37.582) with respect to `"SoC"`, leading to a negative value of the estimated ICER (-1103.645). However, a considerable degree of uncertainty is associated with the mean incremental estimates as indicated by the \(95\%\) credible intervals which include \(0\) for both outcomes.

### 11.1 Probabilistic sensitivity analysis

`missingHE` automatically generates and stores a few standard Probabilistic Sensitivity Analysis (PSA) measures into a list named `cea` within the `"missingHE"` model object. A series of built-in functions are also available from the package `BCEA` to visually display PSA results derived from a model fitted in `missingHE`. For example, the following commands may be used

    > BCEA::ceplane.plot(sm1_mar$cea, graph = "ggplot2")
    > BCEA::ceac.plot(sm1_mar$cea, graph = "ggplot2")

to generate Figure 6 and Figure 7, which show the cost-effectiveness plane and acceptability curve plots obtained from the PSA results stored in `sm1_mar$cea`.

Figure 6: Inspecting probabilistic sensitivity analysis using BCEA built-in functions, for example in terms of cost-effectiveness plane.

Figure 7: Inspecting probabilistic sensitivity analysis using BCEA built-in functions, for example in terms of cost-effectiveness acceptability curve.

Both graphs are generated based on \(1000\) posterior samples and suggest a favourable cost-effectiveness assessment of `"MenSS"` vs `"SoC"`. In the cost-effectiveness plane, shown in Figure 6, most of the incremental estimates fall in the bottom-right quadrant (where `"MenSS"` is cheaper and more effective than `"SoC"`) and are associated with ICER estimates that are lower than \(k=25,000\) (shaded area). In the cost-effectiveness acceptability curve, shown in Figure 7, estimates of the probability of cost-effectiveness for `"MenSS"` are close to \(1\) for most values of the willingness to pay \(k\).