# Introduction

This R-package `moewishart` provides maximum likelihood estimation (MLE) and Bayesian estimation for the Wishart mixture model and the Wishart mixture-of-experts (MoE-Wishart) model. It implements four different inference algorithms for the two model:

- mixture model of Wishart distributions:
  - EM algorithm for finding the MLE;
  - Bayesian approach using Gibbs-within-MH sampling algorithm.
- Mixture-of-Expert model, in which the gating probabilities depend on covariates:
  - EM-MoE algorithm for finding the MLE;
  - Bayesian-MoE approach using a Gibbs-within-MH sampling algorithm.

## Installation

Install the latest released version from CRAN:

    install.packages("moewishart")

Install the latest development version from GitHub:

    # library("devtools")
    devtools::install_github("zhizuio/moewishart")

## Example

Data simulation from a MoE-Wishart model:

- Sample size \(n = 200\)
- Dimension of the Wishart distribution \(p = 2\)
- Number of latent components \(K = 3\)
- \(q=3\) gating covariates \(\mathbf X = [x_{ij}] \in \mathbb R^{n\times q}\), \(x_{ij}\sim\text{N}(0,1)\), \(i=1,...,n\), \(j=1,...,q\)
- Fixed covariate effects \(\boldsymbol\beta=[\boldsymbol\beta_1,...,\boldsymbol\beta_K] \in \mathbb R^{q\times K}\), with \(\boldsymbol\beta_{K}=0\)
- Probabilities of subpopulations \(\boldsymbol\pi = [\pi_{ik}] \in \mathbb R^{n\times q}\), \(\pi_{ik} = \exp(\mathbf X_i\boldsymbol\beta_k) / \sum_{l=1}^K\exp(\mathbf X_i\boldsymbol\beta_l)\), \(k=1,...,K\)
- Degrees of freedom \(\boldsymbol\nu = (\nu_1,\nu_2,\nu_3) = (8, 12, 3)\)
- Scale matrices of the Wishart distribution \(\Sigma_1\), \(\Sigma_2\), \(\Sigma_3 \in \mathbb R^{p\times p}\)
- Data \(S_i \sim \pi_{i1}\text{Wishart}(\nu_1, \Sigma_1) + \pi_{i2}\text{Wishart}(\nu_2, \Sigma_2) + \pi_{i3}\text{Wishart}(\nu_3, \Sigma_3)\)

### 1. Working model: Bayesian MoE-Wishart model

    library(moewishart)

    n <- 200 # number of subjects
    p <- 2 # dimension of covariance matrix
    set.seed(123) # fix coefficients of underlying MoE model
    Xq <- 3
    K <- 3
    betas <- matrix(runif(Xq * K, -2, 2), nrow = Xq, ncol = K)
    betas[, K] <- 0

    # simulate data
    dat <- simData(n, p,
      Xq = 3, K = 3, betas = betas,
      pis = c(0.35, 0.40, 0.25),
      nus = c(8, 12, 3)
    )

    # fit Bayesian MoE-Wishart model
    set.seed(123)
    fit <- moewishart(
      dat$S,
      X = cbind(1, dat$X), K = 3,
      mh_sigma = c(0.2, 0.1, 0.2), # RW-MH variances (length K)
      mh_beta = c(0.3, 0.3), # RW-MH variances (length K-1)
      niter = 3000, burnin = 1000
    )

Posterior means for degrees of freedom (DoF) of Wishart distributions:

    burnin <- 1000
    nu_mcmc <- fit$nu[-c(1:burnin), ]
    colMeans(nu_mcmc)

    ## [1]  8.574911 14.397351  3.310689

True DoF:

    dat$nu # true nu

    ## [1]  8 12  3

Posterior means for scale matrices of Wishart distributions:

    MoE_Sigma <- Reduce("+", fit$Sigma) / length(fit$Sigma)
    MoE_Sigma

    ## , , 1
    ##
    ##           [,1]      [,2]
    ## [1,] 0.5197160 0.2103881
    ## [2,] 0.2103881 0.7470847
    ##
    ## , , 2
    ##
    ##           [,1]      [,2]
    ## [1,] 1.7637949 0.5540576
    ## [2,] 0.5540576 1.3244947
    ##
    ## , , 3
    ##
    ##            [,1]       [,2]
    ## [1,]  4.1115070 -0.1267705
    ## [2,] -0.1267705  3.0385263

Posterior means for gating coefficients:

    beta_mcmc <- fit$Beta_samples[-c(1:burnin), , ]
    apply(beta_mcmc, c(2, 3), mean)

    ##            [,1]        [,2] [,3]
    ## [1,] -0.3656861 -0.08024419    0
    ## [2,] -0.9526224  2.24956385    0
    ## [3,]  1.7609922  2.40287152    0
    ## [4,] -0.4953755 -2.56072719    0

### 2. Working model: Bayesian Wishart mixture model

    # fit Bayesian Wishart mixture model
    set.seed(123)
    fit2 <- mixturewishart(
      dat$S,
      K = 3,
      mh_sigma = c(0.2, 0.1, 0.2), # RW-MH variances
      niter = 3000, burnin = 1000
    )

Posterior means for subpopulation probabilities:

    colMeans(fit2$pi[-c(1:burnin), ])

    ## [1] 0.2690425 0.5088864 0.2220712

Posterior means for DoF of Wishart distributions:

    colMeans(fit2$nu[-c(1:burnin), ])

    ## [1]  7.986113 12.153338  3.284252

### 3. Working model: MoE-Wishart model via EM algorithm

    # fit MoE-Wishart model via EM alg.
    set.seed(123)
    fit3 <- moewishart(
      dat$S,
      X = cbind(1, dat$X), K = 3,
      method = "em",
      niter = 3000
    )

    ## Iter   1  loglik = -2079.322610
    ## Iter   2  loglik = -1998.694495
    ## Iter   3  loglik = -1985.659443
    ## Iter   4  loglik = -1947.223842
    ## Iter   5  loglik = -1899.666938
    ## Iter   6  loglik = -1878.233062
    ## Iter   7  loglik = -1861.702657
    ## Iter   8  loglik = -1851.548347
    ## Iter   9  loglik = -1847.342510
    ## Iter  10  loglik = -1845.497390
    ## Iter  11  loglik = -1844.693281
    ## Iter  12  loglik = -1844.360139
    ## Iter  13  loglik = -1844.220804
    ## Iter  14  loglik = -1844.160033
    ## Iter  15  loglik = -1844.132434
    ## Iter  16  loglik = -1844.119493
    ## Iter  17  loglik = -1844.113254
    ## Iter  18  loglik = -1844.110247
    ## Iter  19  loglik = -1844.108705
    ## Iter  20  loglik = -1844.107915
    ## Iter  21  loglik = -1844.107527
    ## Iter  22  loglik = -1844.107312
    ## Iter  23  loglik = -1844.107207
    ## Iter  24  loglik = -1844.107148
    ## Iter  25  loglik = -1844.107116
    ## Iter  26  loglik = -1844.107098
    ## Iter  27  loglik = -1844.107088
    ## Iter  28  loglik = -1844.107082
    ## Iter  29  loglik = -1844.107080
    ## Iter  30  loglik = -1844.107079
    ## Iter  31  loglik = -1844.107077
    ## Iter  32  loglik = -1844.107077
    ## Converged by loglik tolerance.

EM estimates for DoF of Wishart distributions:

    fit3$nu

    ## [1]  7.515417 13.987158  3.274665

EM estimates for Wishart scale matrices:

    fit3$Sigma

    ## [[1]]
    ##           [,1]      [,2]
    ## [1,] 0.5591113 0.2324429
    ## [2,] 0.2324429 0.8148737
    ##
    ## [[2]]
    ##           [,1]      [,2]
    ## [1,] 1.7665723 0.5567668
    ## [2,] 0.5567668 1.3336367
    ##
    ## [[3]]
    ##            [,1]       [,2]
    ## [1,]  4.3139885 -0.1886288
    ## [2,] -0.1886288  3.0983710

EM estimates for gating coefficients:

    fit3$Beta

    ##             comp1      comp2 comp3
    ## [1,] -0.006270492  0.1302039     0
    ## [2,] -0.798302303  2.0340525     0
    ## [3,]  1.598530103  2.2399293     0
    ## [4,] -0.510585695 -2.3465248     0

### 4. Working model: Wishart mixture model via EM algorithm

    # fit Wishart mixture model via EM alg.
    set.seed(123)
    fit4 <- mixturewishart(
      dat$S,
      K = 3,
      method = "em",
      niter = 3000
    )

    ## Running 3 initialization restarts...
    ##   -> Restart 1: Loglik = -2011.91
    ##   -> Restart 2: Loglik = -1989.16
    ##   -> Restart 3: Loglik = -1989.16
    ## Iter  10 | Loglik: -1953.0326 | Nu: 5, 7.26, 5
    ## Iter  20 | Loglik: -1952.7924 | Nu: 5.15, 7.05, 5.21
    ## Iter  30 | Loglik: -1930.6036 | Nu: 4.21, 7.13, 8.27
    ## Iter  40 | Loglik: -1902.9306 | Nu: 3.08, 7.78, 10.79
    ## Iter  50 | Loglik: -1902.6288 | Nu: 3, 7.76, 11.11
    ## Iter  60 | Loglik: -1902.6133 | Nu: 2.99, 7.71, 11.21
    ## Iter  70 | Loglik: -1902.6102 | Nu: 2.99, 7.69, 11.26
    ## Iter  80 | Loglik: -1902.6095 | Nu: 2.99, 7.68, 11.28
    ## Iter  90 | Loglik: -1902.6093 | Nu: 3, 7.68, 11.3
    ## Iter 100 | Loglik: -1902.6092 | Nu: 3, 7.68, 11.3
    ## Iter 110 | Loglik: -1902.6092 | Nu: 3, 7.68, 11.31
    ## Converged at iteration 116

EM estimates for DoF of Wishart distributions:

    fit4$nu

    ## [1]  2.995383  7.682819 11.309040

EM estimate for Wishart scale matrices:

    fit4$Sigma

    ## [[1]]
    ##           [,1]     [,2]
    ## [1,]  4.048859 -1.10103
    ## [2,] -1.101030  2.79641
    ##
    ## [[2]]
    ##           [,1]      [,2]
    ## [1,] 0.5582012 0.2515063
    ## [2,] 0.2515063 0.8151752
    ##
    ## [[3]]
    ##           [,1]      [,2]
    ## [1,] 2.0930529 0.6273737
    ## [2,] 0.6273737 1.5706730

## Reference

Mai TT, Zhao Z (2026). Mixture-of-experts Wishart model for covariance matrices with an application to Cancer drug screening. arXiv:2602.13888.