D.24 Answers: CIs for odds ratios
Answers to exercises in Sect. 25.9.
Answer to Exercise 25.1:
1. \(99/62 = 1.596774\); about 1.60.
2. \(216/115 = 1.878261\); about 1.88.
3. \(1.596774/1.878261 = 0.850\), as in the output.
4. A few ways; for example:
For every 100 men with a smooth scar,
about 85 women with a smooth scar.
5. (Graph not shown, but use a stacked or side-by-side bar chart.)
6. Table D.4.
7. Exact 95% CI for the OR, from the output: 0.576 to 1.255.
8. If study repeated study many times
(with the same numbers of men and women),
about 95% of the CIs would contain population OR.
In practice:
population OR is probably between 0.576 and 1.255.
Odds with smooth scars | Percentage with smooth scars | Sample size | |
---|---|---|---|
Women: | 1.60 | 61.5% | 161 |
Men: | 1.88 | 65.3% | 331 |
Odds ratio: | 0.850 |
Answer to 25.2: The output can be interpreted in one of two ways (Sect. 25.2):
- Odds are the odds of swimming at the beach; OR compares these odds between those without an ear infection, to those with an ear infection.
- Odds are the odds of not having an ear infection; OR compares these odds for beach swimmers to non-beach swimmers.
Answer to 25.3:
1. Table D.5.
2. Table D.6.
3. OR: Odds of a 1800-hr turbine getting a fissure is 0.352 times
the odds of a 3000-hr turbine getting a fissure.
4. CI from 0.13 to 1.14.
Plausible values for the population OR that may have produced the sample OR
likely to be between these values.
Fissures | No fissures | Total | |
---|---|---|---|
About 1800 hours | 7 | 73 | 80 |
About 3000 hours | 9 | 33 | 42 |
Total | 16 | 106 | 122 |
Odds with fissures | Percentage with fissures | Sample size | |
---|---|---|---|
About 1800 hours | 0.0959 | 8.75% | 80 |
About 3000 hours | 0.2727 | 21.43% | 42 |
Odds ratio | 0.352 |
Answer to Exercise 25.4:
Odds of no rainfall (non-positive SOI): \(14/40 = 0.35\).
Odds of no rainfall (negative SOI): \(7/53 = 0.1320755\).
Required OR is
\(0.35/0.1320755 = 2.65\), as in output.
95% CI from 0.979 to 7.174.
Answer to Exercise 25.5:
The 95% CI is from 0.151 to 0.408.
The OR of not wearing a hat, comparing males to females
(malesless likely to be not wearing a hat;
rewording, males more likely to be wearing a hat).