We define a new function log_lik() rather than a stats::logLik() method because (in addition to the conceptual difference) the documentation for logLik() states that the return value will be a single number, whereas log_lik() returns a matrix. See the log_lik.stanreg() method in the rstanarm package for an example of defining a method.

log_lik(object, ...)

## Arguments

object The object to use. Arguments passed to methods. See the methods in the rstanarm package for examples.

## Value

log_lik() methods should return a $$S$$ by $$N$$ matrix, where $$S$$ is the size of the posterior sample (the number of draws from the posterior distribution) and $$N$$ is the number of data points.

# See help("log_lik", package = "rstanarm")