1. What is Incidence Density?

Incidence density is a measure of the instantaneous rate of occurrence of new cases of a disease in a population. It accounts for varying follow-up times among study participants and is expressed as cases per person-time.

2. How Does the Calculator Work?

The calculator uses the incidence density formula:

Where:

• \( Cases \) — Number of new cases observed
• \( Person-Time \) — Total observation time across all participants

Explanation: This calculation provides the rate at which new cases occur in a population, accounting for the total time that each person was observed and at risk of developing the condition.

3. Importance of Incidence Density Calculation

Details: Incidence density is particularly useful in cohort studies where participants have different follow-up times. It provides a more accurate measure of disease occurrence than simple incidence proportion when follow-up times vary significantly.

4. Using the Calculator

Tips: Enter the number of new cases observed and the total person-time of observation. Person-time is typically measured in person-years but can be expressed in other time units as appropriate for your study.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between incidence density and incidence proportion?
A: Incidence density is a rate (cases per person-time) while incidence proportion is a proportion (cases per persons at risk). Incidence density accounts for varying observation times.

Q2: How is person-time calculated?
A: Person-time is the sum of the time that each person in the study population was at risk of developing the outcome and under observation.

Q3: When should I use incidence density?
A: Use incidence density when study participants have different follow-up times, which is common in cohort studies with staggered entry or varying withdrawal times.

Q4: Can incidence density be greater than 1?
A: Yes, incidence density can be greater than 1, especially when using small time units. For example, 10 cases per 5 person-years equals 2 cases per person-year.

Q5: How should I report incidence density results?
A: Always report the time unit used (e.g., cases per person-year, cases per 1000 person-days) and consider providing a confidence interval to indicate precision.