← Probability methods
Two-way tables, expected value & risk
Restrict first for "given". E = n × p. Compare risks with a ratio.| Cafeteria | Packed | Total | |
|---|---|---|---|
| Junior | 90 | 110 | 200 |
| Senior | 120 | 80 | 200 |
| Total | 210 | 190 | 400 |
P(senior | packed) = 80/190 ≈ 0.42 — restrict to the packed column FIRST. P(packed | senior) = 80/200 = 0.40. Not the same thing!
Reading a two-way table for a claim
- Convert every raw count to a proportion of ITS OWN group total before comparing across groups.
- A relative risk near 1 means the groups are similarly likely; well above/below 1 signals a genuine difference.
Practice for this standard covers the whole topic — flashcards and quiz.