For epidemiology: compute the incidence rate from new cases and person-time (per 1,000 / 10,000 / 100,000 person-years), the cumulative incidence (risk) from the population at risk, and rate ratios comparing exposed vs unexposed groups.
| Rate (per multiplier) | Cases | Person-time |
|---|
Numbers computed by this tool's own engine (oracle-verified):
| Measure | Result |
|---|---|
| Incidence rate | 500 per 100,000 person-years (raw 0.005 per person-year) |
| Cumulative incidence | 5.00% risk (50 of 1000 at risk) |
| Exposed rate | 400 per 100,000 person-years |
| Unexposed rate | 200 per 100,000 person-years |
| Rate ratio | 2.000 (exposed vs unexposed) |
| Rate difference | 200 per 100,000 person-years |
Example: 50 new cases in 10,000 person-years; 50 new cases among 1,000 people at risk; exposed 20 cases / 5,000 person-years vs unexposed 10 cases / 5,000 person-years.
Incidence rate = new cases / total person-time at risk (CDC person-time rate). Scaled for reporting: rate × multiplier, e.g. ×100,000 for "per 100,000 person-years".
Cumulative incidence (risk) = new cases / population at risk at the start of the period — a proportion between 0 and 1.
Rate ratio = exposed rate / unexposed rate; rate difference = exposed rate − unexposed rate (scaled to the same multiplier).
Note: unlike cumulative incidence, an incidence rate is not a proportion — because its denominator is person-time, it can exceed 1 per unit time when many cases occur quickly.
Incidence rate = number of new cases / total person-time at risk. For example, 50 new cases in 10,000 person-years gives 50 / 10,000 = 0.005 per person-year, which is usually scaled for reporting: 0.005 x 100,000 = 500 per 100,000 person-years. This matches the CDC definition of the person-time rate.
Both count new cases, but their denominators differ. Incidence rate divides new cases by the total person-time at risk (it handles people entering and leaving follow-up), and is not a proportion — it can exceed 1 per unit time. Cumulative incidence (incidence proportion, or risk) divides new cases by the population at risk at the start of the period in a closed cohort, and is always a proportion between 0 and 1. For example, 50 new cases among 1,000 people followed for a year is a cumulative incidence of 50 / 1,000 = 5%.
Incidence counts only NEW cases arising during a period; prevalence counts ALL cases present (new plus pre-existing). The CDC states the difference in the numerators: incidence uses the number of new cases of disease during the interval, while point prevalence uses the number of current cases (new and pre-existing) at one point in time. Incidence measures the speed of disease occurrence; prevalence measures the disease burden in the population.
Person-time is the sum of the time each person was observed and at risk of the disease, typically expressed in person-years. If 10 people are followed for 2 years each, that is 20 person-years; someone followed for only 6 months contributes 0.5 person-years. Only disease-free time counts — once a person develops the disease (for incidence of a first event) they stop contributing.
A rate ratio of 2.0 means the incidence rate in the exposed group is twice the rate in the unexposed group (exposed / unexposed = 2.0). Rate ratio > 1 means a higher rate in the exposed group, < 1 means a lower rate, and exactly 1 means equal rates. Also look at the rate difference: a ratio of 2.0 could mean 2 vs 1 or 200 vs 100 per 100,000 person-years — very different absolute impacts.
Because raw incidence rates are usually tiny decimals (e.g. 0.005 per person-year) that are hard to read and compare. Multiplying by 1,000, 10,000 or 100,000 just rescales the same rate into whole numbers. The choice of multiplier does not change the result — 0.005 per person-year = 5 per 1,000 = 500 per 100,000 person-years.
Free tool by NRTechStudio
Explore similar health tools and calculators. 100% free, accurate, and runs completely in your browser without sign-up.
2 taps, no typing needed — diagnostics are sent automatically.