Randy Neil Biostats 03: Biostatistics Summary Step 1 - The Basics USMLE

USMLE purpose: Master the “guaranteed points” setup for sensitivity, specificity, predictive values, study design, odds ratio, relative risk, cutoffs, prevention, power, and standard deviation.

How to use this page

Start by memorizing the 2×2 test box.
Review the formula table, then practice identifying the metric from the stem wording.
Use the worked examples for cutoff, power, prevention, and SD questions.
Open the transcript only if you want the original explanation.

Episode metadata

One-liner

This is the core Randy Neil “Biostats Basics” episode: draw the box, label the study design, recognize the formula, and do the arithmetic without overthinking.

Core formulas / rules

Concept	Rule	Stem clue
Sensitivity	TP / (TP + FN)	“Among people with disease, how many test positive?”
Specificity	TN / (TN + FP)	“Among people without disease, how many test negative?”
PPV	TP / (TP + FP)	Positive test → chance truly diseased
NPV	TN / (TN + FN)	Negative test → chance truly disease-free
Case-control	Odds ratio	Starts with disease vs no disease
Cohort	Relative risk	Starts with exposure and follows outcomes
Power	1 − β	Probability of detecting a true difference
Normal distribution	68–95–99.7 rule	Mean ± SD questions

Exam pattern recognition

“False negatives?” → fill in the 2×2 table first.
“Older students with lower scores compared to younger students” → decide whether odds ratio or relative risk based on study design.
“Move cutoff from B to A” → identify whether false positives or false negatives shrink.
“Checking blood pressure at a health fair” → secondary prevention.
“Probability due to chance” → p-value.
“15% probability of concluding no difference when there is one” → β; power = 1 − β.

High-yield visual: prevention levels

Level	Timing	Examples
Primary	Before disease occurs	Vaccination, counseling, risk reduction
Secondary	Early/asymptomatic disease	Screening tests, BP check, colonoscopy
Tertiary	Established disease	Treatment, rehab, complication prevention