Characteristics | Sample of study population | Sample of unexposed pregnant women, and census of exposed population (from study population). | |
---|---|---|---|
Example 1 | Example 2 | Example 3 | |
Study population size of eligible pregnant women (Study population) | 20.000 | 20.000 | 20.000 |
Sample size | 2.700 | 800 (ratio 1:3) | 1.200 (ratio 1:5) |
Exposed (sample) | 27 (1%)a | ||
Unexposed (sample) | 2673 | ||
Exposed (from population identified by surveillance systems) | – | 200 (1%)a exposed | 200 (1%)a exposed |
Unexposed (from population identified by surveillance systems) | – | 600–6 exposed (1%) = 594 unexposed | 1.000–10 exposed (1%) = 990 unexposed |
Ratio exposed:unexposed | 1:99 (as the study population) | 1:3 (defined by investigator) | 1:5 (defined by investigator) |
Expected cases among exposed (rounded numbers) | 1 (5%)a | 10 (5%)a | 10 (5%)a |
Expected cases among unexposed (round numbers) | 7 (0.25%)a | 1 (0.25%)a | 2 (0.25%)a |
Total expected cases (round numbers) | 8 | 11 | 12 |
Type 1 error | 0.05 | 0.05 | 0.05 |
Study Powerb | 59% | 95% | 97% |