Moment matching algorithm for updating a loo object when Pareto k estimates are large.

loo_moment_match(x, ...)

# S3 method for default
loo_moment_match(
x,
loo,
post_draws,
log_lik_i,
unconstrain_pars,
log_prob_upars,
log_lik_i_upars,
max_iters = 30L,
k_threshold = 0.7,
split = TRUE,
cov = TRUE,
cores = getOption("mc.cores", 1),
...
)

## Arguments

x A fitted model object. Further arguments passed to the custom functions documented above. A loo object to be modified. A function the takes x as the first argument and returns a matrix of posterior draws of the model parameters. A function that takes x and i and returns a matrix (one column per chain) or a vector (all chains stacked) of log-likelihood draws of the ith observation based on the model x. If the draws are obtained using MCMC, the matrix with MCMC chains separated is preferred. A function that takes arguments x, and pars and returns posterior draws on the unconstrained space based on the posterior draws on the constrained space passed via pars. A function that takes arguments x and upars and returns a matrix of log-posterior density values of the unconstrained posterior draws passed via upars. A function that takes arguments x, upars, and i and returns a vector of log-likelihood draws of the ith observation based on the unconstrained posterior draws passed via upars. Maximum number of moment matching iterations. Usually this does not need to be modified. If the maximum number of iterations is reached, there will be a warning, and increasing max_iters may improve accuracy. Threshold value for Pareto k values above which the moment matching algorithm is used. The default value is 0.5. Logical; Indicate whether to do the split transformation or not at the end of moment matching for each LOO fold. Logical; Indicate whether to match the covariance matrix of the samples or not. If FALSE, only the mean and marginal variances are matched. The number of cores to use for parallelization. This defaults to the option mc.cores which can be set for an entire R session by options(mc.cores = NUMBER). The old option loo.cores is now deprecated but will be given precedence over mc.cores until loo.cores is removed in a future release. As of version 2.0.0 the default is now 1 core if mc.cores is not set, but we recommend using as many (or close to as many) cores as possible. Note for Windows 10 users: it is strongly recommended to avoid using the .Rprofile file to set mc.cores (using the cores argument or setting mc.cores interactively or in a script is fine).

## Value

The loo_moment_match() methods return an updated loo object. The structure of the updated loo object is similar, but the method also stores the original Pareto k diagnostic values in the diagnostics field.

## Details

The loo_moment_match() function is an S3 generic and we provide a default method that takes as arguments user-specified functions post_draws, log_lik_i, unconstrain_pars, log_prob_upars, and log_lik_i_upars. All of these functions should take .... as an argument in addition to those specified for each function.

## Methods (by class)

• default: A default method that takes as arguments a user-specified model object x, a loo object and user-specified functions post_draws, log_lik_i, unconstrain_pars, log_prob_upars, and log_lik_i_upars.

## References

Paananen, T., Piironen, J., Buerkner, P.-C., Vehtari, A. (2020). Implicitly Adaptive Importance Sampling. preprint arXiv:1906.08850

loo(), loo_moment_match_split()

## Examples

# See the vignette for loo_moment_match()