[jmgirard]

compare_performance() computes LOOIC stacking weights from the pointwise elpd_loo values of each model with cbind(). When the models were fit to different numbers of observations, cbind() recycled the shorter column, and loo::stacking_weights() returned wrong weights. For 32 rows against 150 rows base R warned "number of rows of result is not a multiple of vector length". For 32 rows against 64 rows there was no warning at all.

An example is brms_1 (32 observations) with brms_4 (150 observations) from insight::download_model(). On main the LOOIC weights are 0.000 and 1.000.

What changes

• If the models with LOOIC have different numbers of pointwise rows, compare_performance() gives NA for all LOOIC weights and, with verbose = TRUE, warns with the counts. Models with the same number of rows keep their weights.
• The "Model Weights" section of the help says that LOOIC weights are NA in this case.

Tests

• brms_1 with brms_4, also with rank = TRUE, with a copy of brms_1 as a third model, and with verbose = FALSE.
• Mocked model_performance() results with 32 and 64 rows, where cbind() recycled without a warning, and with a third model that has no LOOIC.
• brms_1 with a copy of itself still gets the weights 0.5 and 0.5, as on main.

The new tests fail on main and pass with this change.

[Copilot]

🟡 Changes recommended

Matching row counts do not ensure pointwise observations are aligned across models.

1 open issue

• Validate observation alignment before computing stacking weights · New

1 file not reviewed

• man/compare_performance.Rd: Generated file

What changed in this PR

This pull request prevents invalid LOOIC stacking weights when models have different pointwise observation counts.

Changes:

• Returns NA weights and warns when observation counts differ.
• Preserves weights for matching counts.
• Adds regression tests, documentation, NEWS, and a version bump.

File	Summary
tests/​testthat/​test-compare_performance.R	Adds regression coverage.
R/​compare_performance.R	Validates pointwise row counts before stacking.
NEWS.md	Documents the bug fix.
man/​compare_performance.Rd	Documents LOOIC weight behavior.
DESCRIPTION	Increments the package development version.

_{Review effort: Lite}