| name | pg-nonparametric |
| description | Run or generate Pingouin code for rank-based non-parametric tests — Mann-Whitney U, Wilcoxon signed-rank, Kruskal-Wallis, Friedman, and Cochran Q — when outcomes are ordinal or parametric assumptions fail. |
PG Nonparametric
Use when the outcome is ordinal, or continuous but non-normal with small n, so a rank-based test is safer than a t test or ANOVA.
Load
Read:
../../references/supervision-gates.md../../references/pingouin-api-quickref.md../../references/apa-output-template.md if writing results.
Decision Rules
- Two independent groups ->
pg.mwu(x, y) (Mann-Whitney U).
- Same participants measured twice / paired ->
pg.wilcoxon(x, y) (signed-rank).
- Three or more independent groups, one factor ->
pg.kruskal(data, dv, between).
- Three or more repeated conditions, continuous/ordinal ->
pg.friedman(data, dv, within, subject).
- Three or more repeated conditions, binary outcome ->
pg.cochran(data, dv, within, subject).
- Follow a significant omnibus with
pg.pairwise_tests(..., parametric=False, padjust="holm").
Required Inputs
- Outcome column and its scale (ordinal or non-normal continuous).
- Grouping (between) or condition (within) column.
- Subject ID for paired/repeated designs.
- Post hoc correction: default
holm.
- Alternative hypothesis: default
two-sided (mwu/wilcoxon only).
Code Patterns
Mann-Whitney U (independent):
x = df.loc[df["group"].eq("A"), "score"]
y = df.loc[df["group"].eq("B"), "score"]
res = pg.mwu(x, y, alternative="two-sided").round(3)
pg.print_table(res)
Wilcoxon signed-rank (paired):
res = pg.wilcoxon(df["pre"], df["post"], alternative="two-sided").round(3)
pg.print_table(res)
Kruskal-Wallis (k independent groups):
res = pg.kruskal(data=df, dv="score", between="group").round(3)
pg.print_table(res)
Friedman (k repeated conditions):
res = pg.friedman(data=df, dv="score", within="condition", subject="id").round(3)
pg.print_table(res)
Cochran Q (binary repeated):
res = pg.cochran(data=df, dv="passed", within="condition", subject="id").round(3)
pg.print_table(res)
Output Checks
mwu/wilcoxon return U_val/W_val, p_val, RBC (rank-biserial), CLES. kruskal returns H, ddof1, p_unc; friedman returns Kendall W, Q, p_unc; cochran returns Q, dof, p_unc. Report the effect size, not just p.
Guardrails
mwu is unpaired only; wilcoxon is paired only — do not swap them.- Kruskal/Friedman/Cochran are omnibus; always follow a significant result with corrected pairwise tests.
- Rank tests compare distributions/ranks, not means; word conclusions accordingly.
- Prefer parametric tests when their assumptions hold (more power).
- End result-bearing answers with one compact S0-S5 audit line.
