Randy Neil Biostats 02: Biostatistics Summary Step 1 - USMLE The Extra Stuff

USMLE purpose: Memorize the second-layer formulas: NNT/NNH, likelihood ratios, attributable risk percent, length-time bias, and Hardy-Weinberg.

How to use this page

Review the formula board before opening the practice answers.
Work the NNT, likelihood ratio, attributable risk, and Hardy-Weinberg examples by hand.
Use the rapid recall toggles for spaced review.
Use the transcript only if you want to verify Randy’s original wording.

Episode metadata

Source / episode info

Field	Details
Episode	Randy Neil Biostats 02
Topic	Extra biostats concepts
Runtime	24 min
Published	2016-04-25
Source	https://www.youtube.com/watch?v=VMI9UuNqoGI

One-liner

This episode is the formula-heavy companion to the basics: know the formulas cold, recognize which formula the stem is asking for, and plug in carefully.

Formula board

Question asks for…	Use this	Shortcut trigger
Number needed to treat	1 / ARR	“How many patients must be treated to prevent one outcome?”
Absolute risk reduction	Control event rate − treatment event rate	Placebo vs treatment table
Positive likelihood ratio	Sensitivity / (1 − specificity)	“Positive LR”
Negative likelihood ratio	(1 − sensitivity) / specificity	“Negative LR”
Attributable risk percent	(RR − 1) / RR	“What percent is attributable to exposure?”
Hardy-Weinberg	p² + 2pq + q² = 1; p + q = 1	Autosomal recessive population frequency

Exam pattern recognition

“How many need to be treated?” → NNT = 1 / ARR.
“Positive likelihood ratio” → sensitivity on top, 1 − specificity on bottom.
“Percent attributable to exposure” → attributable risk percent.
“Slow-growing / benign cases detected by screening” → length-time bias.
“Rare autosomal recessive disease frequency” → start with q².

How to use this page

Source / episode info

One-liner

Formula board

Exam pattern recognition

High-yield bias distinction