Part 2 · Epidemiology I — Measures and Observational Designs
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| Fraction | Definition | Numerator in denominator? | Units | Range |
|---|---|---|---|---|
| Ratio | x / y, any two quantities | Not necessarily | Often unitless | 0 to ∞ |
| Proportion | a / (a + b) | Yes | Unitless | 0 to 1 (0–100%) |
| Rate | events / (population at risk × time) | Yes, with time | Per unit time (e.g., per person‑year) | 0 to ∞ |
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