Journal of Behavioral Data Science, 2024, 4 (1), 1–24.
DOI: https://doi.org/10.35566/jbds/tongliu

## A Tutorial on Bayesian Linear Regression with Compositional Predictors Using JAGS

University of Virginia, Charlottesville, VA 22904, USA
xt8b@virginia.edu
Abstract This tutorial offers an exploration of advanced Bayesian methodologies for compositional data analysis, specifically the Bayesian Lasso and Bayesian Spike-and-Slab Lasso (SSL) techniques. Our focus is on a novel Bayesian methodology that integrates Lasso and SSL priors, enhancing both parameter estimation and variable selection for linear regression with compositional predictors. The tutorial is structured to streamline the learning process, breaking down complex analyses into a series of straightforward steps. We demonstrate these methods using R and JAGS, employing simulated datasets to illustrate key concepts. Our objective is to provide a clear and comprehensive understanding of these sophisticated Bayesian techniques, preparing readers to adeptly navigate and apply these methods in their own compositional data analysis endeavors.

Keywords: Bayesian analysis · Compositional data · Lasso · Spike and Slab Lasso.

### 1 Introduction

Compositional data, also referred to as ipsative data or fractional data, are non-negative components that represent fractions of a finite whole. Examples include percentages that sum to 100, hours in a day that sum to 24, expenses on different items that sum to the total budget, etc. Since compositional data contain relative information within the whole, analyzing such data is valuable to better differentiate ratings and diminish response biases. Compositional data analysis can be employed whether all components contributing to the total have been quantified, or merely a subset of them (Chastin, Palarea-Albaladejo, Dontje, & Skelton2015). Although compositional data appear in many fields, including but not limited to geology, biology, economics, medicine, and psychology, the analysis of this type of data as predictors has received relatively little attention due to the unique challenge caused by the constant-sum constraint in compositional data, notably a statistical issue known as “exact collinearity”. With the collinearity issue, traditional statistical methods may yield biased parameter estimates, inflated standard errors, and reduced statistical power (e.g., Belsley, Kuh, & Welsch1980).

A common strategy to address the exact collinearity problem of compositional data is to use transformations. Since compositional data reside in a simplex and mainly carry relative information, a log-ratio transformation is typically employed, based on the Aitchison geometry (Aitchison1986). Suitable log-ratio representations include the additive-log-ratio (alr) transformation, the centered log-ratio (clr) transformation, and the isometric-log-ratio (ilr) transformation (Aitchison1986Egozcue, Pawlowsky-Glahn, Mateu-Figueras, & Barceló-Vidal2003). However, the application of transformation-based methods for compositional data analysis presents certain drawbacks. First, transforming data containing zero values poses a challenge since log zero is undefined. To circumvent the singularity, it is typical to replace zero values with a small predetermined value. This approach, however, can introduce undesired bias and potentially produce misleading results, particularly when dealing with highly sparse data containing a substantial number of zeros (Aitchison & Bacon-Shone1984Palarea-Albaladejo & Martin-Fernandez2013). Second, methods based on transformations struggle to provide clear interpretations, given their inability to fully disentangle the dependency among predictors. This means that a shift in a single predictor value inherently triggers alterations in other predictor values. As a result, it can distort the original data structure, thereby complicating the interpretation of the transformed data.

Because of the limitations of the transformation techniques, researchers have been seeking alternative strategies to handle compositional data and yield interpretable model estimation results (e.g., G. Li, Li, & Chen2023). It is shown in the literature that Bayesian methods are promising as even with exact collinearity, posteriors of the model parameters are well-defined. Although Bayesian compositional data analysis is sensitive to priors of the parameters, our previous findings demonstrated that Bayesian Lasso and Bayesian Spike-and-Slab Lasso (SSL) provide reasonable estimation results.

In this tutorial, we focus on linear regression and offer a hands-on exploration of both the Bayesian Lasso and Bayesian SSL methods to handle compositional predictors. By integrating Bayesian methods within the Lasso and SSL frameworks, we highlight the flexibility and robustness of our approach, thus enhancing the accuracy of parameter estimation and facilitating a more comprehensive understanding of the relationships among the compositional predictors and the outcome. The readers will learn how to implement the Bayesian Lasso and Bayesian SSL techniques using R (R Core Team2023), equipped with the Just Another Gibbs Sampler (JAGS) program (Plummer2003). The structure of the tutorial is organized as follows. We begin with a preliminaries section that provides a detailed introduction to compositional data, highlighting its complexities and challenges. We also explore penalized methods, focusing specifically on the Lasso and SSL techniques. This leads to a section dedicated to Bayesian methods, where we establish the theoretical basis for the Bayesian Lasso and Bayesian SSL methodologies. The subsequent sections demonstrate the implementation of these Bayesian methods in JAGS. We conclude the tutorial with a summary of insights and implications drawn from our discussion.

### 2 Preliminaries

#### 2.1 Linear regression modeling of compositional predictors

This tutorial focuses on a linear regression model with compositional predictors, given by:

\begin {equation} y_{i}=\beta _{0}+\mathbf {x}_{i}^{T}\boldsymbol {\beta }+\varepsilon _{i},\quad \varepsilon _{i}\sim \mathcal {N}\left (0,\sigma ^{2}\right ),\label {eq:Reg} \end {equation} where $$y_{i}$$ represents the outcome for the $$i$$th subject ($$i=1,\ldots ,n$$, $$n$$ is the total sample size), $$\beta _{0}$$ is the intercept, $$\mathbf {x}_{i}=(x_{i1},\ldots ,x_{ip})^{T}$$ is a vector of compositional predictors with $$p$$ components for the $$i$$th subject, $$\boldsymbol {\beta }=\left (\beta _{1},\ldots ,\beta _{p}\right )^{T}$$ represents a $$p$$-dimensional vector of unknown regression coefficients, and $$\varepsilon _{i}$$ is the random error following a normal distribution with mean 0 and unknown variance $$\sigma ^{2}$$. According to the definition, compositional data with $$p$$ components refers to non-negative numbers whose sum is a constant. Namely, $$x_{i1}>0,\ldots ,x_{ip}>0$$, and \begin {equation} \sum _{j=1}^{p}x_{ij}=c,\label {eq:Compositional data} \end {equation} where $$c$$ is a constant. Without loss of generality, we set $$c$$ at 1. Under the sum-to-one constraint, the following issues may arise:

(1) The components of compositional data fall within the $$(0,1)$$ interval, which can complicate the prediction of the dependent variable due to the limited range of possible outcomes.

(2) The sum-to-one constraint leads to an exact collinearity problem among predictors, which poses a significant challenge to standard regression techniques, as they typically operate under the assumption of predictor independence.

(3) Real-world compositional data frequently manifest a high degree of sparsity, i.e., a large share of the components could be zero. This introduces additional intricacies to the analysis, as discerning whether a zero value signifies a genuine absence of the component or merely results from measurement limitations can be challenging.

In this tutorial, our focus lies on scenarios typified where a proportion of $$\boldsymbol {\beta }$$ are either zero or sufficiently small to render the majority of potential predictors insignificant in the analysis. Within this framework, we have two main goals: (i) discerning the influential predictors, and (ii) precisely estimating the magnitude of their corresponding effects.

#### 2.2 Penalized least squares regression

To address collinearity, frequentist approaches often use a penalized likelihood to obtain sparse estimates of the regression coefficient. These methods involve adding a penalty term into the model to regulate the magnitude of the coefficients and effectively shrinking them towards zero to reduce their variance. Similar to ordinary least squares (OLS) estimation, penalized regression methods estimate the regression coefficients $$\beta$$ by minimizing the penalized residual sum of squares, given by:

\begin {equation} \min _{\beta }\left \{ \sum _{i=1}^{n}\left (y_{i}-\beta _{0}-\mathbf {x}_{i}^{T}\boldsymbol {\beta }\right )^{2}+\operatorname {pen}_{\lambda }(\boldsymbol {\beta })\right \} .\label {eq:Penalized least squares regression} \end {equation} The penalized term $$\operatorname {pen}_{\lambda }(\boldsymbol {\beta })$$ can maintain all features while diminishing the coefficients’ magnitude in the model. A widely used approach is the Least Absolute Shrinkage and Selection Operator (Lasso), introduced by Tibshirani (1996), which incorporates an $$L1$$ penalty term into the objective function, expressed as $$\operatorname {pen}_{\lambda }(\boldsymbol {\beta })=\lambda \sum _{j=1}^{p}|\beta _{j}|$$ . The parameter $$\lambda$$ can be tuned to set the shrinkage level: the higher the $$\lambda$$ is, the more coefficients are shrunk to zero. This approach aims to enhance predictive precision and identify a concise group of significant predictors.

However, in high-dimensional scenarios, traditional Lasso may arbitrarily select only one predictor from a group of highly correlated variables, potentially leading to biased estimations and reduced interpretability (Zou & Hastie2005). Additionally, Lasso employs a constant penalty for all coefficients and could unintentionally result in the inclusion of irrelevant predictors (Friedman, Hastie, & Tibshirani2010). These issues are particularly pertinent in compositional data analysis, where each predictor represents a fraction of the total. The presence of irrelevant predictors can skew these fractions, potentially leading to inaccurate interpretations (W. Lin, Shi, Feng, & Li2014).

To reduce the bias of Lasso, Ročkov’a and George (2018) introduced the Spike-and-Slab Lasso (SSL), an advanced technique that incorporates penalized likelihood in the Bayesian framework. The SSL methodology has become a prevalent tool for handling diverse statistical problems due to its inherent flexibility and versatility. For example, Tang, Shen, Zhang, and Yi (2017b) and Tang et al. (2018) used SSL within the framework of generalized linear models to enhance the accuracy of disease outcome prediction and improve gene detection efficiency. In the context of multivariate regression, Deshpande, Ročková, and George (2019) developed a multivariate SSL to offer an effective solution for simultaneous predictor selection and effect estimation. Moreover, Z. Li, McCormick, and Clark (2019) proposed an innovative class of priors for Bayesian inference using SSL in multiple Gaussian graphical models, demonstrating its ability to facilitate simultaneous self-adaptive shrinkage and model selection. Also, the integration of SSL into Cox proportional hazards models by Tang, Shen, Zhang, and Yi (2017a) has significantly advanced survival data analysis by offering a nuanced understanding of time-dependent risks.

In general, penalized least squares methods are often effective for identifying important predictors and managing collinearity in regression analysis. However, when applied to compositional data, these methods in the frequentist framework still require a transformation of compositional predictors, which struggle to provide straightforward interpretations. To address this, we propose a shift towards a Bayesian framework. This shift is grounded in the equivalence between the frequentist penalized regression approach and the Bayesian approach, where a normal likelihood combined with specific priors leads to a posterior distribution akin to the penalized likelihood. In our Bayesian framework, we integrate Lasso and SSL priors, enhancing the interpretability of results with compositional data. We introduce Bayesian Lasso and Bayesian SSL below.

### 3 Bayesian Lasso and Bayesian SSL

#### 3.1 Bayesian Lasso

Tibshirani (1996) suggested that Lasso estimates can be interpreted as posterior modes when regression coefficients have independent and identically distributed Laplace (double-exponential) priors. Thus, the prior for the regression coefficients $$\boldsymbol {\beta }$$ given the error variance $$\sigma ^{2}$$ with hyperparameter $$\lambda$$ is defined as

\begin {equation} p\left (\boldsymbol {\beta }\mid \sigma ^{2}\right )=\prod _{j=1}^{p}\frac {\lambda }{2\sqrt {\sigma ^{2}}}\exp \left \{ \frac {-\lambda \left |\beta _{j}\right |}{\sqrt {\sigma ^{2}}}\right \} .\label {eq:Bayesian Lasso} \end {equation} Park and Casella (2008) pointed out that this Laplace distribution is equivalent to a continuous mixture of Gaussian distributions (Andrews & Mallows1974). Specifically, the expression for the Laplace distribution as a scale mixture of normals (with an exponential mixing density) is given by

\begin {equation} \begin {aligned}\frac {\lambda }{2}\exp (-\lambda |\beta |)= & \int _{0}^{\infty }\frac {1}{\sqrt {2\pi \tau ^{2}}}\exp \left (-\beta ^{2}/2\tau ^{2}\right )\times \frac {\lambda ^{2}}{2}\exp \left (-\frac {\lambda ^{2}\tau ^{2}}{2}\right )\mathrm {d}\tau ^{2}\end {aligned} ,\label {eq:Laplace distribution} \end {equation} where $$\tau ^{2}$$ serves as a scaling factor. This suggests the following hierarchical representation of the full model:

\begin {equation} \begin {gathered}{y_{i}\mid \beta _{0},\mathbf {x}_{i},\boldsymbol {\beta },\sigma ^{2}\sim \mathcal {N}\left (\beta _{0}+\mathbf {x}_{i}^{T}\boldsymbol {\beta },\sigma ^{2}\right )},\\ \boldsymbol {\beta }\mid \sigma ^{2},\tau _{1}^{2},\ldots ,\tau _{p}^{2}\sim \mathcal {N}\left (\mathbf {0},\sigma ^{2}\boldsymbol {D}_{\tau }\right ),\\ \beta _{0}\sim \mathcal {N}\left (0,100\right )\\ \sigma ^{2},\tau _{1}^{2},\ldots ,\tau _{p}^{2}\sim p\left (\sigma ^{2}\right )\prod _{j=1}^{p}\frac {\lambda ^{2}}{2}\exp \left \{ \frac {-\lambda ^{2}\tau _{j}^{2}}{2}\right \} . \end {gathered} \label {eq:Hierarchical posterior} \end {equation} In this model, $$\boldsymbol {D}_{\tau }$$ is a diagonal matrix with diagonal elements $$\tau _{1}^{2},\ldots ,\tau _{p}^{2}$$. The exponential distribution for $$\tau _{j}^{2}$$ reflects the Lasso penalty, with $$\lambda$$ controlling the amount of penalization. For $$\sigma ^{2}$$, we choose a non-informative scale-variant prior, such as $$p\left (\sigma ^{2}\right )=1/\sigma ^{2}$$. Or, to ensure model conjugacy, an inverse-gamma prior for $$\sigma ^{2}$$ can be utilized.

In contrast to traditional Lasso that necessitates manual adjustment of its penalty hyperparameter, the Bayesian Lasso facilitates the empirical estimation of the shrinkage degree. This capability significantly improves the model’s effectiveness in handling complexity and mitigating issues of collinearity. However, despite these benefits, Ročková and George (2016) identified certain shortcomings in the Bayesian Lasso, particularly its limited capacity to accommodate sparsity and to rectify estimation bias. Further, as Ghosh, Tang, Ghosh, and Chakrabarti (2013) indicates, it tends to insufficiently shrink less significant coefficients while excessively shrinking more important ones. The following subsection will explore how the Bayesian SSL addresses these specific challenges.

#### 3.2 Bayesian SSL

Unlike Lasso, which imposes uniform shrinkage across all regression coefficients $$\boldsymbol {\beta }$$, the SSL employs a mixture prior structure. Each coefficient $$\beta _{j}$$ comes from either a Laplacian "spike" centered around zero or a broader Laplacian "slab". The hierarchical prior over $$\boldsymbol {\beta }$$ and the latent indicator variables $$\boldsymbol {\gamma }=\left (\gamma _{1},\ldots ,\gamma _{p}\right )$$ is given by: \begin {equation} \begin {aligned} & p(\boldsymbol {\beta }\mid \gamma )=\prod _{j=1}^{p}\left [\left (1-\gamma _{j}\right )\psi _{0}\left (\beta _{j}\right )+\gamma _{j}\psi _{1}\left (\beta _{j}\right )\right ],\quad \gamma \sim p(\boldsymbol {\gamma }),\end {aligned} \label {eq:spike and slab Lasso} \end {equation} where $$\gamma =\left (\gamma _{1},\ldots ,\gamma _{p}\right )^{\prime }$$, with each $$\gamma _{i}$$ being a binary variable, is an intermediate vector, indexing the $$2^{p}$$ conceivable models within the framework. Here, $$\psi _{0}\left (\beta \right )=\left (\lambda _{0}/2\right )e^{-\left |\beta \right |\lambda _{0}}$$ is the “spike” and $$\psi _{1}\left (\beta \right )=\left (\lambda _{1}/2\right )e^{-\left |\beta \right |\lambda _{1}}$$ is the “slab” $$\left (\lambda _{1}\ll \lambda _{0}\right )$$ . This two-point mixture of Laplace distributions will be referred to as the SSL priors. The flexibility of the model space prior $$p(\gamma )$$ substantially broadens the scope of these priors. It allows for the tailoring of $$p(\beta )$$ towards preferred configurations of $$\gamma$$, enhancing the model’s ability to align with specific, desirable configurations. For our analysis, we focus on of the following form:

\begin {equation} p(\boldsymbol {\gamma }\mid \theta )=\prod _{j=1}^{p}\theta ^{\gamma _{j}}(1-\theta )^{1-\gamma _{j}},\quad \text { and }\quad \theta \sim \operatorname {Beta}(a,b), \end {equation} where $$\theta =\mathrm {P}\left (\gamma _{j}=1\mid \theta \right )$$ represents the prior expected fraction of substantial $$\beta _{j}$$. This hyperparameter $$\theta$$ plays a pivotal role in the mixture prior, as it balances the influence of the spike and slab components by controlling the mixture probabilities for each coefficient $$\beta _{j}$$. Additionally, $$a$$ and $$b$$ serve as hyperparameters in the Beta distribution, further shaping the overall behavior of the model.

In the Bayesian SSL framework, our sampling methodology for the Spike-and-Slab Lasso posterior relies on the Stochastic Search Variable Selection (SSVS) algorithm (George & McCulloch1993). This approach interprets the Laplace distribution as a scale mixture of Gaussians defined by penalty parameters $$\lambda _{j}>0$$. The mixing distribution is exponential, with the rate parameter set at $$\lambda _{j}^{2}/2$$, following the approach outlined in Park and Casella (2008). Thus, regression with SSL priors takes the following hierarchical form:

\begin {equation} \begin {aligned} & y_{i}\mid \beta _{0},\mathbf {x}_{i},\boldsymbol {\beta },\sigma ^{2}\sim \mathcal {N}\left (\beta _{0}+\mathbf {x}_{i}^{T}\boldsymbol {\beta },\sigma ^{2}\right ),\\ & \beta _{0}\sim \mathcal {N}\left (0,100\right ),\\ & \boldsymbol {\beta }\mid \boldsymbol {\tau }\sim \mathcal {N}\left (\mathbf {0},\sigma ^{2}\boldsymbol {D}_{\tau }\right ),\\ & {\tau _{j}^{2}\sim \operatorname {Exp}\left (\frac {\lambda _{j}^{2}}{2}\right ),}\\ & \lambda _{j}=\gamma _{j}\lambda _{1}+\left (1-\gamma _{j}\right )\lambda _{0},\\ & \gamma _{j}\mid \theta \sim \operatorname {Bernoulli}(\theta )\quad \text { with }\quad \theta \sim \operatorname {Beta}(a,b),\\ & {\sigma ^{2}\sim InvGamma(0.001,0.001)}\text {.} \end {aligned} \end {equation} In the model, $$\boldsymbol {D}_{\tau }$$ represents a diagonal matrix, with its diagonal comprising the elements $$\tau _{1}^{2},\ldots ,\tau _{p}^{2}$$. The hyperparameters $$\lambda _{1}$$ and $$\lambda _{0}$$ are used for computing $$\lambda _{j}$$, and are typically chosen based on prior knowledge, with $$\lambda _{1}$$ being considerably smaller than $$\lambda _{0}$$ (expressed as $$\lambda _{1}\ll \lambda _{0}$$ ). The model’s spike-and-slab component is encapsulated by the latent variables $$\gamma _{1},\ldots ,\gamma _{p}$$, each adhering to a Bernoulli distribution characterized by the parameter $$\theta$$, symbolized as $$\operatorname {Bernoulli}(\theta )$$. Furthermore, the parameter $$\theta$$ itself is modeled using a Beta distribution, determined by the shape parameters $$a$$ and $$b$$. Again for $$\sigma ^{2}$$, we can choose a non-informative scale-variant prior, such as $$p\left (\sigma ^{2}\right )=1/\sigma ^{2}$$. Or, to ensure model conjugacy, an inverse-gamma prior for $$\sigma ^{2}$$ can be utilized.

The Bayesian SSL introduces a more sophisticated model for variable selection. It combines the shrinkage property of the Lasso with a spike-and-slab prior, allowing for a more flexible and nuanced approach. This hierarchical structure not only enhances the model’s flexibility but also aids in capturing the underlying complexities of the data. This methodology is particularly effective in dissecting the intricate structure of compositional data, offering a comprehensive understanding of the underlying relationships within the dataset.

Next, we delve into the practical implementation of the Bayesian Lasso and Bayesian SSL using the rjags package.

### 4 Bayesian Lasso implementation in JAGS

In this section, we introduce how to use the rjags package in the R environment (Plummer2003) to call JAGS and carry out Bayesian compositional analysis. JAGS, short for Just Another Gibbs Sampler, can be installed from the following link: https://mcmc-jags.sourceforge.io/.

#### 4.1 Software

The following chunk of code loads the necessary packages for our entire tutorial. If you haven’t installed these packages previously, you will need to install them first using install.packages(). In our Bayesian analysis, we utilize rjags for interfacing R with JAGS, while coda assists in summarizing MCMC outputs. These tools collectively streamline the implementation and evaluation of Bayesian models in R.

library(rjags)           # rjags library allows R to interface with JAGS
library(coda)            # coda package provides tools for summarizing and visualizing MCMC output
library(ggmcmc)          # ggmcmc is used for diagnostics of MCMC chains and plots
library(MASS)            # MASS for generating multivariate normal data
library(Matrix)          # Matrix package is for matrix computations, particularly for sparse matrices
library(MCMCpack)        # Commonly used for generating samples from a Dirichlet distribution in R
}

#### 4.2 Compositional data generation

For illustration, we generate a compositional data matrix with $$n=50$$ subjects and $$p=10$$ components. Two commonly used compositional data generation methods are introduced below.

##### Method 1: Generate normally distributed data and normalize to compositional data.

We start by generating a matrix X of random variables, following normal distributions. Each row of X represents observations from one subject, and each column represents a component. Then, compositional data can be obtained by dividing each element in X by its corresponding row sum. After the conversion, elements in each row sum to 1.

p <- 10  # Define the number of variables
N <- 50  # Define the number of observations

# Initialize a matrix Z with N rows and p columns to hold the generated normal data
X <- matrix(rnorm(N * p), nrow = N, ncol = p)

# Normalize each row so that they sum to 1 (compositional data)
X <- X / rowSums(X)
##### Method 2: Generate data from Dirichlet distribution.

Dirichlet distribution is a common choice for generating compositional data because it inherently produces a vector of values that sum to 1. It is a multivariate generalization of the beta distribution. Compositional predictors can be directly generated from a Dirichlet distribution as below.

p <- 10  # Define the number of variables
N <- 50  # Define the number of observations

alpha <- rep(1, p)  # Specify hyperparameters for the Dirichlet distribution

# Initialize an empty matrix X with N rows and p columns to hold the generated data
X <- matrix(0, nrow = N, ncol = p)

# Fill the matrix X with random values drawn from the Dirichlet distribution
for (i in 1:N) {
X[i,] <- rdirichlet(1, alpha)
}

After compositional predictors are generated, we generate the outcomes based on Equation (1), with the intercept $$\beta _{0}=1$$, coefficient values $$\boldsymbol {\beta }$$ = $$(-2,-1.5,-1,0,$$ $$1,1.5,2,0,0,0)^{T}$$ and error variance $$\sigma ^{2}=0.01$$. A subset of the generated data is presented in Table 1.

q <- 6   # Define the number of non-zero regression coefficients

# Define the true regression coefficients; first q coefficients are non-zero, rest are zero
beta_scenario <- c(-2, -1.5, -1, 0, 1, 1.5, 2, rep(0, p - q-1))

# Generate the outcome variable y based on equation (1)
y <- 1 + X %*% beta_scenario + rnorm(N, sd = 0.1)

#### 4.3 Data analysis

For illustration purpose, we use one dataset generated using Method 2 to demonstrate the implementation of Bayesian Lasso below.

Step 1: Define JAGS model

In this step, we define a Bayesian Lasso regression model using JAGS. The model is specified in a string format and includes components for the likelihood, priors for the regression coefficients $$\beta$$, and other model parameters.

model_string_Lasso <- "
model {
# Likelihood
for (i in 1:N) {
y[i] ~ dnorm(mu[i], pre_sig2) # normal distribution for y
mu[i] <- beta0+sum(beta[] * X[i,]) # model for the mean
}

# Prior for beta0 (intercept)
beta0 ~ dnorm(0, 1.0E-2)  # Assuming a weakly informative prior

# Prior for sigma2
pre_sig2 ~ dgamma(0.01,0.01)        #gamma prior for precision
sigma2 <- 1/pre_sig2

# Priors for beta coefficients
for (j in 1:p) {
# Using the scale mixture representation of the Laplace distribution:
beta[j] ~ dnorm(0, tau_beta[j])
tau_beta[j] <- 1 / (sigma2 * tau_sq[j])

# Prior for tau_sq[j] (related to the Lasso penalty)
tau_sq[j] ~ dexp(lambda^2 / 2) # follows an exponential distribution with rate lambda^2 / 2
}

}
"

Step 2: Specify initial values and prepare data

In this step, we define initial values for the parameters in the Bayesian Lasso regression model. It is recommended to use estimates from a standard least squares regression as initial values for the $$\lambda$$ parameter in the Bayesian Lasso model.

In JAGS, users have the choice of using user-specified initial values or default values from random number generators (RNGs). Our explanations will cover both methods. For illustration, we let RNGs determine initial values for Bayesian Lasso and use user-defined initial values for Bayesian SSL.

The base module in JAGS includes four distinct Random Number Generators (RNGs), each identified by the following names: “base::Wichmann-Hill”, “base::Marsaglia-Multicarry”, “base::Super-Duper”, and “base::Mersenne-Twister”. To initialize the RNG of your choice, simply specify its name along with a seed value (for example, a seed of 111), as illustrated in the subsequent code example:

# Initial values for the parameters

# Automatically generated initial values
inits_lasso <- list(".RNG.name"="base::Wichmann-Hill", ".RNG.seed"=111)

We then prepare the data in a list format to be compiled with the model in JAGS.

# Data preparation
# N: the number of observations.
# p: the number of predictors.
# X: the design matrix.
# y: the response variable (converted to a vector using as.vector(y)).

lambda <- p * sqrt(var(lm(y ~ X- 1)$residuals)) / sum(abs(lm(y ~ X- 1)$coefficients))
data_lasso <- list(N = N, p = p, X= X, y = as.vector(y), lambda = lambda)

Step 3: Fit the model via JAGS

In our Bayesian Lasso model, we use a Markov chain Monte Carlo (MCMC) algorithm. Note that the number of chains is set at 1 (n.chains = 1) for simplicity, but it can be easily set at a larger value to check between-chains convergence, which is crucial for a reliable model. We run the MCMC for 5000 iterations (update(fit, 5000)) as a burn-in period, and then use 20,000 more iterations (n.iter = 20000) to get a detailed view of the parameters.

fit <- jags.model(
textConnection(model_string_Lasso), # Model specification.
data = data_lasso, # The data.
inits=inits_lasso,  # The initial values for the model parameters.
n.chains = 1, # The number of Markov chains.
n.adapt = 1000  # The number of adaptation iterations
)
params <- c("beta0","beta","sigma2")
update(fit, 5000) # burn-in
samples <- coda.samples(fit, variable.names = params, n.iter = 20000)
#save results into model.res
model.res <- as.mcmc(do.call(rbind,samples))

Step 4: Convergence diagnostic via traceplots and Geweke tests

Prior to examining the results of parameter estimates, we assess the convergence of the Markov chains using two common diagnostic tools: traceplots and Geweke tests.

The traceplots of the Markov chains for $$\boldsymbol {\beta }$$ are displayed in Figure1. Clearly, all chains converged, indicating effective sampling from the posterior distributions for all coefficients $$\beta _{1}$$ through $$\beta _{10}$$. Geweke test results are given below. With Geweke statistics falling between the -1.96 to 1.96 range, there is no evidence against convergence, suggesting that the model estimation is reliable for interpretation.

# Trace plot
race_plot <- traceplot(samples)

# geweke tests
geweke.diag(model.res)

Fraction in 1st window = 0.1
Fraction in 2nd window = 0.5

beta[1]  beta[2]  beta[3]  beta[4]
-0.14747 -0.06480 -0.08991 -0.02971
beta[5]  beta[6]  beta[7]  beta[8]
-0.16094  0.14410 -0.16851 -0.06328
beta[9] beta[10]
-0.16662 -0.04245

Step 5: Plot regression coefficients estimation for interpretation

In this step, we focus on visualizing the estimates of the beta coefficients for better interpretation and understanding.

# Convert the results from JAGS into a format that ’ggmcmc’ can use.
ggmcmc_object <- ggs(model.res)

# Creates a caterpillar plot which is used for visualizing the posterior distributions of the parameters.
ggs_caterpillar(D = ggmcmc_object, family = "beta")

The coefficient dot plots of the Markov chains for $$\beta$$ are in Figure 2. The black points represent the mean estimates of the coefficients. The black lines indicate the range of these estimates, extending from the 2.5% percentile to the 97.5% percentile. Specifically, the coefficients $$\beta _{1}$$, $$\beta _{2}$$, and $$\beta _{3}$$ are notably negative, with their 95% credible intervals excluding zero. This indicates a substantial negative influence on the outcome variable as the corresponding predictor proportions increase. Coefficients $$\beta _{5}$$, $$\beta _{6}$$, and $$\beta _{7}$$ are positive, with credible intervals also excluding zero, suggesting a significant positive effect on the outcome variable with increasing proportions of these predictors. The coefficient $$\beta _{4}$$, $$\beta _{8}$$, $$\beta _{9}$$, and $$\beta _{10}$$ are close to zero with credible intervals that encompass zero, indicating no discernible impact on the outcome variable.

Step 6: Summarize posterior MCMC samples

In this step, we summarize the model estimation results from the MCMC procedure conducted in JAGS. The summary() function provides a comprehensive overview of the posterior distributions for each parameter in the model. From the results, the model appears to perform well in estimating the true values of the regression coefficients. The empirical means are close to the true values, and the true values generally fall within the 95% credible intervals of the estimates. Due to sampling errors of one set of simulated data, there is bias. But the small bias (-0.18, -0.01, -0.07, -0.29, 0.17, 0.01, 0.22, -0.17, 0.01, -0.06) for the coefficients $$\boldsymbol {\beta }$$ demonstrates the effectiveness of Bayesian Lasso in capturing the underlying true values of the coefficients.

summary(samples)
Iterations = 6001:26000
Thinning interval = 1
Number of chains = 1
Sample size per chain = 20000

1. Empirical mean and standard deviation for each variable,
plus standard error of the mean:

Mean     SD Naive SE Time-series SE
beta[1]  -2.18754 0.5981 0.002115        0.07902
beta[2]  -1.51041 0.5877 0.002078        0.07892
beta[3]  -1.07185 0.6009 0.002124        0.07484
beta[4]  -0.29220 0.5845 0.002067        0.07816
beta[5]   1.17219 0.5913 0.002090        0.08011
beta[6]   1.51421 0.6062 0.002143        0.07398
beta[7]   2.22883 0.6215 0.002197        0.07185
beta[8]  -0.17333 0.5853 0.002069        0.08046
beta[9]   0.01159 0.5794 0.002048        0.07991
beta[10] -0.06262 0.5853 0.002069        0.07988
beta0     1.05053 0.5703 0.002016        0.08483

2. Quantiles for each variable:

2.5%     25%       50%      75%   97.5%
beta[1]  -3.34861 -2.5699 -2.196116 -1.83954 -0.9258
beta[2]  -2.65227 -1.8809 -1.519331 -1.17487 -0.2676
beta[3]  -2.24595 -1.4559 -1.082853 -0.70673  0.1647
beta[4]  -1.44032 -0.6641 -0.291097  0.04308  0.9430
beta[5]   0.02786  0.7949  1.163895  1.51551  2.4180
beta[6]   0.33887  1.1301  1.502785  1.86480  2.8035
beta[7]   1.02289  1.8299  2.220343  2.60242  3.5282
beta[8]  -1.31749 -0.5395 -0.179262  0.15835  1.0615
beta[9]  -1.11653 -0.3470  0.003906  0.33893  1.2432
beta[10] -1.20086 -0.4322 -0.067443  0.26883  1.1846
beta0    -0.15954  0.7427  1.055721  1.40493  2.1579

### 5 Bayesian SSL Implementation in JAGS

Step 1: Define JAGS model

In this step, we define a Bayesian SSL regression using JAGS syntax. The model is specified in a string format and includes components for the likelihood, priors for the regression coefficients, and other model parameters.

model_string_SSL <- "
model {
# -----------------
# Likelihood
# -----------------
for (i in 1:N) {
y[i] ~ dnorm(mu[i], pre_sig2)
mu[i] <- beta0 + sum(beta[] * X[i,])
}

# Priors for beta and reparameterization for Laplace prior
for (i in 1:p) {
# Prior for gamma (Bernoulli distribution)
gamma[i] ~ dbern(theta)   # Binary variable following Bernoulli distribution.

# Definition of lambda
lambda[i] <- gamma[i] * lambda1 + (1 - gamma[i]) * lambda0   #Mixing two lambda values based on gamma

# Prior for beta (Normal distribution with mean 0 and precision tau)
tau_sq[i] ~ dexp(lambda[i]^2 / 2)
beta[i] ~ dnorm(0, tau_beta[i])
tau_beta[i] <- 1 / (sigma2 * tau_sq[i])
}

theta ~ dbeta(a, b)  # Beta prior for theta (Bernoulli parameter)
beta0 ~ dnorm(0, 0.01)  # Normal prior for the intercept
pre_sig2 ~ dgamma(0.01,0.01)  # Precision follows a gamma distribution
sigma2 <- 1/pre_sig2 # Definition of sigma squared (variance) as the reciprocal of precision
}
"

Step 2: Specify initial values and prepare data

Here, in the Bayesian SSL approach for illustration, we set initial values for model parameters using user-specified initial values.

# Initial values
# Define Initial Values
inits <- list(list(gamma = rep(1, p),  beta0 = rnorm(1, 0, 1), pre_sig2 = 1 , theta = 0.5, beta = rep(0, p) ))
# Prepare data list
data_list_SSL <- list(
y = as.vector(y),  # Response variable
X = X,  # Matrix of predictors
N = N,  # Number of observations
p = p,  # Number of predictors
a = 2,  # Hyperparameter for the Beta distribution prior on theta
b = p,  # Hyperparameter for the Beta distribution prior on theta
lambda1 = 0.1,  # Hyperparameter for the lambda calculation
lambda0 = 4 # Hyperparameter for the lambda calculation
)

Step 3: Fit the Bayesian SSL model via JAGS

The code below serves to run the model and manage the adaptation and burn-in phases of the MCMC procedure.

fit <- jags.model(
textConnection(model_string_SSL),  # Model specification.
data = data_list_SSL,  # The data.
inits = inits_SSL,  # The initial values for the model parameters.
n.chains = 1,  # The number of Markov chains.
n.adapt = 1000  # The number of adaptation iterations
)
update(fit, 2000)
params <- c("beta","beta0","sigma2")

#run 30000 iterations after the burn-in priord
samples <- coda.samples(fit, params, n.iter = 30000)

#save results into model.res
model.res <- as.mcmc(do.call(rbind,samples))

Step 4: Convergence diagnostic via traceplots and Geweke tests

The model convergence is again assessed using two common diagnostic tools: traceplots and Geweke tests.

# Trace plot
race_plot <- traceplot(samples)

# geweke tests
geweke.diag(model.res)

Fraction in 1st window = 0.1
Fraction in 2nd window = 0.5

beta[1]  beta[2]  beta[3]  beta[4]  beta[5]  beta[6]  beta[7]  beta[8]
-0.2644  -0.2936  -0.3114  -0.3153  -0.4079  -0.2915  -0.3601  -0.5842
beta[9] beta[10]    beta0
-0.3861  -0.6112   0.3281

The traceplots of the Markov chains for $$\boldsymbol {\beta }$$ using Bayesian SSL are given in Figure 3. Again, both traceplots and Geweke tests imply that all Markov chains converged, thus parameter estimates can be trusted.

Step 5: Plot regression coefficients estimation for interpretation

In this step, we focus on visualizing the estimates of the beta coefficients from our Bayesian model for better interpretation and understanding.

# Convert the results from JAGS into a format that ’ggmcmc’ can use.
ggmcmc_object <- ggs(model.res)

# Creates a caterpillar plot which is used for visualizing the posterior distributions of the parameters.
ggs_caterpillar(D = ggmcmc_object, family = "beta")

The coefficient dot plots of the Markov chains for $$\boldsymbol {\beta }$$ are provided in Figure 4. The posterior estimates for the regression coefficients are depicted in the plot. The interpretation from the Bayesian SSL model is the same as that from the Bayesian Lasso model.

Step 6: Summarize posterior MCMC samples

In this step, we summarize the model estimation results. The parameter estimates from Bayesian SSL are similar to those from the Bayesian Lasso model. Thus, the detailed interpretations are omitted here.

summary(samples)
Iterations = 3001:33000
Thinning interval = 1
Number of chains = 1
Sample size per chain = 30000

1. Empirical mean and standard deviation for each variable,
plus standard error of the mean:

Mean      SD  Naive SE Time-series SE
beta[1]  -1.879920 0.23686 0.0013675      0.0050494
beta[2]  -1.164649 0.17559 0.0010138      0.0031632
beta[3]  -0.806203 0.18310 0.0010571      0.0033090
beta[4]  -0.008885 0.05826 0.0003363      0.0017450
beta[5]   1.091903 0.15091 0.0008713      0.0036028
beta[6]   1.582677 0.14494 0.0008368      0.0017249
beta[7]   2.005218 0.17140 0.0009896      0.0035410
beta[8]   0.006645 0.04939 0.0002851      0.0012593
beta[9]  -0.001426 0.03078 0.0001777      0.0002378
beta[10]  0.001519 0.03137 0.0001811      0.0002364
beta0     0.933072 0.06327 0.0003653      0.0020230

2. Quantiles for each variable:

2.5%      25%        50%      75%    97.5%
beta[1]  -2.33530 -2.03941 -1.8821989 -1.72026 -1.41082
beta[2]  -1.50748 -1.28124 -1.1653645 -1.04836 -0.81613
beta[3]  -1.16813 -0.92793 -0.8068750 -0.68623 -0.44117
beta[4]  -0.15571 -0.02149 -0.0022969  0.01305  0.07433
beta[5]   0.79652  0.99090  1.0918781  1.19291  1.38692
beta[6]   1.29616  1.48652  1.5834631  1.67931  1.86601
beta[7]   1.66909  1.89193  2.0064159  2.11760  2.34457
beta[8]  -0.07222 -0.01275  0.0022081  0.02050  0.11890
beta[9]  -0.06681 -0.01646 -0.0007303  0.01404  0.06191
beta[10] -0.06310 -0.01421  0.0007147  0.01645  0.06935
beta0     0.80760  0.89115  0.9327129  0.97590  1.05674

### 6 Discussion

This tutorial has introduced the Bayesian Lasso and Bayesian SSL methods as powerful tools for analyzing regression models with compositional predictors. These Bayesian techniques offer distinct advantages, including the ability to incorporate prior knowledge, manage a large number of covariates, and handle exact collinearity and sparsity in data. We provided a detailed walkthrough of the Bayesian Lasso and SSL methods implemented in JAGS, designed to equip researchers and analysts with the practical skills needed to apply Bayesian techniques to compositional data. While the tutorial has focused on compositional predictors, it is important to note that including continuous predictors alongside compositional ones is a common scenario in practice. For analyses involving continuous predictors alongside compositional ones, readers can easily modify the JAGS code structure presented here and add other predictors. Additionally, while JAGS is our chosen software for demonstration, alternatives like Stan may offer different benefits, and we include a Stan code example in the Appendix for further exploration. In summary, this tutorial offers a practical foundation for applying Bayesian methods to regression with compositional predictors, with guidance for extending the analysis to more complex scenarios.

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### Appendix

In the appendix of this tutorial, we present an implementation of the Bayesian Lasso using the Stan programming language. Note that Stan does not support discrete parameters, which is a prerequisite for implementing the Bayesian SSL model. As such, we currently are only able to provide the Bayesian Lasso implementation within the Stan framework.

stan_model_code<- "
data {
int<lower=0> N; // Number of observations
int<lower=0> p; // Number of predictors
matrix[N, p] X; // Predictor matrix
vector[N] y; // Response variable
real<lower=0> lambda; // Lasso penalty parameter
}

parameters {
real beta0; // Intercept
vector[p] beta; // Coefficients for predictors
real<lower=0> sigma; // Standard deviation of errors
vector<lower=0>[p] tau_sq; // Scale parameter for Laplace prior
}

model {

// Likelihood
for (i in 1:N) {
y[i] ~ normal(beta0 + dot_product(beta, X[i]), sigma);
}

// Priors
beta0 ~ normal(0, 100); // Weakly informative prior for the intercept
sigma ~ cauchy(0, 2.5); // Cauchy prior for sigma
for (j in 1:p) {
tau_sq[j] ~ exponential(lambda^2 / 2); // Exponential prior for tau_sq
}

// Conditional prior for beta using scale mixture representation
for (j in 1:p) {
beta[j] ~ normal(0, sqrt(tau_sq[j] * sigma^2)); // Laplace prior represented as scale mixture
}
}

"
y<-as.vector(y)
# Initial values for the parameters
lambda <- p * sqrt(var(lm(y ~ X - 1)$residuals)) / sum(abs(lm(y ~ X - 1)$coefficients))

# Compile the Stan model
sm <- stan_model(model_code = stan_model_code)

# Run the initial model
fit <- sampling(sm, data = list(N = N, p = p, X = X, y = y, lambda = lambda),iter = 2000, chains = 1, warmup = 500, thin = 1)