Randy Neil Biostats 09: USMLE Step 1 - Effect Modulation vs. Confounding; t-test, ANOVA

USMLE purpose: Separate confounding from effect modification and choose the right statistical test for means/tables.

How to use this page

Start with the confounding vs effect modification table.
Review the statistical test guide until you can match the data type to the test.
Use the rapid recall toggles before looking at the practice explanations.
Use the transcript only if you want Randy’s original wording.

Episode metadata

One-liner

Confounding affects both exposure and outcome; effect modification changes the outcome relationship in a subgroup and should be reported, not “controlled away.”

Confounding vs effect modification

Concept	What it affects	What to do
Confounding	Exposure and outcome	Control/adjust for it
Effect modification	Outcome relationship differs by subgroup	Report stratified effects

Statistical test quick guide

Data / comparison	Test	Shortcut
Two means	Two-sample t-test	Two groups, continuous outcome
Three or more means	ANOVA	“A-NO-VA” = think multiple means
Proportions / 2×2 table	Chi-square	Box/table comparison
Pooled data from studies	Meta-analysis	Combines studies; increases power
NNT	1 / ARR	How many treated to prevent one event?

Exam pattern recognition

Smoking changes medication → DVT relationship but not medication exposure → effect modification.
Smoking relates to alcohol use and pancreatic cancer → confounding.
Compare vitamin D means in two groups → t-test.
Compare categories in a 2×2 table → chi-square.
RR/OR confidence interval crosses 1 → not significant.

How to use this page

One-liner

Confounding vs effect modification

Statistical test quick guide

Exam pattern recognition

Common trap