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Randy Neil Biostats 19: USMLE Step 1 - Viewer Requested Biostats Questions #6
Episode Notes
USMLE purpose: Solve bell-curve count questions by converting standard deviations into percentages and then into patient counts.
How to use this page
- Identify the mean and SD.
- Locate the cutoff relative to the mean.
- Convert the region under the curve into a percent.
- Multiply by the sample size.
Episode metadata
| Field | Details |
| Episode | Randy Neil Biostats 19 |
| Topic | Viewer requested questions #6 |
| Runtime | 10 min |
| Published | 2021-03-16 |
| Source | Open YouTube video |
One-liner
If the cutoff is 1 SD above the mean , then everything below it is 50% + 34% = 84% .
Bell curve shortcut
| Region | Percent | Use |
| Mean to +1 SD | 34% | Half of 68% |
| Below +1 SD | 84% | 50% below mean + 34% |
| Above +1 SD | 16% | Remaining tail |
| Above +2 SD | 2.5% | Tail beyond 95% |
Worked example
Mean = 82 kg, SD = 4 kg, sample size = 1000.
- 86 kg is 1 SD above the mean.
- Percent below +1 SD = 84%.
- 84% of 1000 = 840 people .
Anki-style rapid recall
What percent is below +1 SD?
84%.
What percent is above +1 SD?
16%.
What percent is above +2 SD?
2.5%.
Practice Question
A study has 1000 people, mean 82, SD 4. How many are expected to weigh less than 86?
- A) 160
- B) 500
- C) 680
- D) 840
Reveal answer & explanation
Answer: D) 840
86 is +1 SD. The area below +1 SD is 84%; 84% of 1000 = 840.