CODEWITHSUNDEEP
countstatisticsdistributionhierarchical-datajags
stefano
stefano

Reputation: 415

How to introduce a random error within a Bayesian count model?

I am trying to model counts distribute across groups in JAGS, but I need to add a random effect for the presence of multiple counts from the same individual.

The model is taken from here enter link description here I am stuck, so I would greatly appreciate any help with this.

# File name "count model.jags"

model {
    #Likelihood
    for (i in 1:nGroups) {
        obs[i] ~ dbin(p[i],n[i])
        p[i] ~ dbeta(a[i],b[i])
        a[i] ~ dgamma(1,0.01)
        b[i] ~ dgamma(1,0.01)
    } 
} # end of model

Upvotes: 0

Views: 75

Answers (1)

web devlopment king
web devlopment king

Reputation: 354

To incorporate random effects for repeated measurements from the same individual in your binomial model, you'll need to add a hierarchical structure. Here's how you can modify your JAGS model:

model {
  # Likelihood
  for (i in 1:nObs) {  # Loop over all observations
    obs[i] ~ dbin(p[i], n[i])
    logit(p[i]) <- beta0 + beta[group[i]] + u[individual[i]]  # Linear predictor with random effect
  }
  
  # Random effects for individuals
  for (j in 1:nIndividuals) {
    u[j] ~ dnorm(0, tau.u)  # Individual random effects
  }
  
  # Group-level effects
  for (k in 1:nGroups) {
    beta[k] ~ dnorm(0, 0.001)  # Group effects
  }
  
  # Hyperpriors
  beta0 ~ dnorm(0, 0.001)      # Global intercept
  tau.u ~ dgamma(0.001, 0.001) # Precision for random effects
  sigma.u <- 1/sqrt(tau.u)     # Convert to standard deviation
}

Your data structure should look like this:

data <- list(
  nObs = length(obs),
  nIndividuals = length(unique(individual_ids)),
  nGroups = length(unique(group_ids)),
  obs = observed_counts,
  n = trials,
  individual = individual_ids,
  group = group_ids
)

This approach properly accounts for within-individual correlation while maintaining your group structure. The random effects capture individual-specific deviations from the group-level effects.

Upvotes: 2

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