Fix Kruskal-Wallis test reporting failure with degenerate cases (one observation per group)

NEWS.md

@@ -3,6 +3,7 @@
 Bug fixes
 
 * Fix `report()` crash when character vector has only one unique value (#578).
+* `report()`: fix failure when reporting Kruskal-Wallis tests with degenerate cases (one observation per group) (#454)
 
 # report 0.6.3
 

tests/testthat/_snaps/windows/report.lmer.new.md

@@ -0,0 +1,45 @@
+# report-lmer
+
+    Code
+      report(m1)
+    Output
+      We fitted a linear mixed model (estimated using REML and nloptwrap optimizer)
+      to predict Reaction with Days (formula: Reaction ~ Days). The model included
+      Days as random effects (formula: ~1 + Days | Subject). The model's total
+      explanatory power is substantial (conditional R2 = 0.80) and the part related
+      to the fixed effects alone (marginal R2) is of 0.28. The model's intercept,
+      corresponding to Days = 0, is at 251.41 (95% CI [237.94, 264.87], t(174) =
+      36.84, p < .001). Within this model:
+
+        - The effect of Days is statistically significant and positive (beta = 10.47,
+      95% CI [7.42, 13.52], t(174) = 6.77, p < .001; Std. beta = 0.54, 95% CI [0.38,
+      0.69])
+
+      Standardized parameters were obtained by fitting the model on a standardized
+      version of the dataset. 95% Confidence Intervals (CIs) and p-values were
+      computed using a Wald t-distribution approximation.
+
+---
+
+    Code
+      report(m2)
+    Message
+      boundary (singular) fit: see help('isSingular')
+      boundary (singular) fit: see help('isSingular')
+    Output
+      We fitted a linear mixed model (estimated using REML and nloptwrap optimizer)
+      to predict Reaction with Days (formula: Reaction ~ Days). The model included
+      mysubgrp as random effects (formula: list(~1 | mysubgrp:mygrp, ~1 | mygrp, ~1 |
+      Subject)). The model's total explanatory power is substantial (conditional R2 =
+      0.71) and the part related to the fixed effects alone (marginal R2) is of 0.28.
+      The model's intercept, corresponding to Days = 0, is at 252.09 (95% CI [232.50,
+      271.69], t(174) = 25.39, p < .001). Within this model:
+
+        - The effect of Days is statistically significant and positive (beta = 10.35,
+      95% CI [8.78, 11.93], t(174) = 12.95, p < .001; Std. beta = 0.53, 95% CI [0.45,
+      0.61])
+
+      Standardized parameters were obtained by fitting the model on a standardized
+      version of the dataset. 95% Confidence Intervals (CIs) and p-values were
+      computed using a Wald t-distribution approximation.
+

tests/testthat/test-report.htest-kruskal.R

@@ -7,3 +7,23 @@ test_that("report.htest-kruskal report", {
     report(x, verbose = FALSE)
   )
 })
+
+test_that("report.htest-kruskal degenerate case (one observation per group)", {
+  # Test case from issue #454: one observation per group causes CI calculation to fail
+  n <- 10
+  set.seed(123)
+  df <- data.frame(a = as.factor(1:n), b = rnorm(n))
+  # Use the formula interface to allow effectsize to retrieve data
+  test <- kruskal.test(b ~ a, data = df)
+
+  # Should not fail and should complete quickly - provide data manually
+  expect_no_error({
+    result <- report(test, data = df, verbose = FALSE)
+  })
+
+  # Result should contain effect size but no CI due to degenerate case
+  result <- report(test, data = df, verbose = FALSE)
+  result_text <- as.character(result)
+  expect_true(grepl("Epsilon squared \\(rank\\) = 1\\.00", result_text))
+  expect_false(grepl("95% CI", result_text))  # No CI should be present
+})