Calculate prevalence ratio from test/exposed and control/unexposed prevalence or 2×2 counts, with absolute difference, relative change, and 95% CI.
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Prevalence Ratio Formula
The calculator uses one core formula in two modes.
PR = P1 / P0
When you enter raw counts from a 2×2 table, the prevalences are calculated first:
P1 = a / n1 P0 = c / n0 PR = (a/n1) / (c/n0)
The 95% confidence interval uses the log transform:
SE(ln PR) = sqrt(1/a - 1/n1 + 1/c - 1/n0) 95% CI = exp(ln(PR) ± 1.96 * SE)
- PR: prevalence ratio
- P1: prevalence in the test or exposed group
- P0: prevalence in the control or unexposed group
- a: cases in the exposed group
- n1: total exposed
- c: cases in the unexposed group
- n0: total unexposed
The Prevalence Values mode divides the two rates you supply, after converting them to a common proportion. The From Counts mode builds the prevalences from the 2×2 cells and adds a confidence interval. If any cell is zero, a 0.5 continuity correction is applied so the variance term stays defined.
Reference Tables
Use these to interpret the number you get back.
| PR value | Meaning |
|---|---|
| PR = 1 | No difference between groups |
| PR = 1.5 | Exposed group has 50% higher prevalence |
| PR = 2.0 | Exposed prevalence is twice the control |
| PR = 0.5 | Exposed prevalence is half the control |
| PR < 1 | Protective association in the exposed group |
| 95% CI pattern | Interpretation |
|---|---|
| Lower bound > 1 | Significantly higher prevalence in the exposed group |
| Upper bound < 1 | Significantly lower prevalence in the exposed group |
| Interval crosses 1 | No significant difference at the 0.05 level |
| Very wide interval | Sample size is too small for a precise estimate |
Example Problems
Example 1: From counts. A cross-sectional survey finds 25 cases of a condition among 200 workers exposed to a chemical and 40 cases among 220 unexposed workers. P1 = 25/200 = 0.125. P0 = 40/220 = 0.182. PR = 0.125 / 0.182 = 0.687. The exposed group actually has about 31% lower prevalence.
Example 2: From rates. A study reports diabetes prevalence of 12.5% in one region and 8.0% in another. PR = 12.5 / 8.0 = 1.56. The first region has 56% higher prevalence than the second.
FAQ
How is the prevalence ratio different from the risk ratio? The math is the same, but a prevalence ratio comes from a cross-sectional study measuring existing cases at one point in time. A risk ratio comes from a cohort study tracking new cases over time.
When should I use the prevalence ratio instead of the odds ratio? Use the prevalence ratio when the outcome is common (above roughly 10%). The odds ratio overstates the effect size when prevalence is high.
Why did the calculator add 0.5 to my counts? If any cell in the 2×2 table is zero, the standard error formula breaks down. Adding 0.5 to each cell is a standard continuity correction that keeps the confidence interval calculable.
Does it matter which group I label as "test" or "exposed"? Yes. Switching the groups inverts the ratio (PR becomes 1/PR). Always state which group is in the numerator when you report the result.
