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Randy Neil Biostats 16: USMLE Step 1 - Viewer Requested Biostats Questions #3
Episode Notes
USMLE purpose: Interpret relative risk and confidence intervals, especially whether a result is significant and what percent risk reduction means.
How to use this page
- Start by checking whether the confidence interval crosses 1.
- For RR below 1, convert the reduction as 1 − RR.
- For RR above 1, interpret increased risk.
- Use the answer toggles to test yourself before reading explanations.
Episode metadata
| Field | Details |
| Episode | Randy Neil Biostats 16 |
| Topic | Viewer requested questions #3 |
| Runtime | 8 min |
| Published | 2021-03-10 |
| Source | Open YouTube video |
One-liner
For relative risk, 1 means no difference ; if the confidence interval includes 1, the result is not statistically significant.
Interpretation table
| Result | Meaning | Exam conclusion |
| RR = 1 | No difference | Exposure does not change risk |
| RR < 1 | Risk is lower | Protective / risk reduction |
| RR > 1 | Risk is higher | Risk increase |
| CI includes 1 | Could be no difference | Not statistically significant |
| CI entirely below or above 1 | Does not include no-effect value | Statistically significant |
Worked example
RR = 0.70 with 95% CI 0.60–0.92.
- 0.70 is below 1 → risk is lower.
- Risk reduction = 1 − 0.70 = 0.30 = 30% reduction .
- CI does not include 1 → statistically significant.
Board Exam Buzzwords
| Phrase | What it means | Answer style |
| RR 0.70 | 30% lower risk | Reduced risk |
| CI 0.65–1.15 | Includes 1 | No significant difference |
| Two treatments similar at endpoint | No superiority | Neither drug superior |
Anki-style rapid recall
What does RR = 1 mean?
No difference in risk between groups.
What does RR = 0.70 mean?
30% relative risk reduction compared with control.
What if a 95% CI for RR includes 1?
The result is not statistically significant.
Practice Questions
Question 1
A treatment has RR 0.70 with 95% CI 0.60–0.92. What is the best interpretation?
- A) 70% risk reduction
- B) 30% risk reduction
- C) No significant difference
- D) 30% risk increase
Reveal answer & explanation
Answer: B) 30% risk reduction
RR 0.70 means risk is 70% of baseline, so the reduction is 1 − 0.70 = 30%. The CI does not cross 1.
Question 2
Drug A vs Drug B has RR 0.90 with 95% CI 0.65–1.15. What should you conclude?
- A) Drug A is superior
- B) Drug B is superior
- C) Neither drug is clearly superior
- D) Drug A reduces risk by 90%
Reveal answer & explanation
Answer: C) Neither drug is clearly superior
The confidence interval includes 1, so the data are compatible with no difference.
Quick Reference Summary
| Item | No-effect value | Crossing rule |
| RR / OR / HR | 1 | If CI crosses 1, not significant |
| Mean difference | 0 | If CI crosses 0, not significant |
📌 Transcript note: The original full transcript is preserved in the source material; the high-yield review above is the primary study tool.