Incidence Rate (Incidence Density)

Definition: Number of new cases occurring during the sum of time each participant was at risk.

Incidence Rate (Incidence Density) = No. new cases Person-time interval × Multiplier

Some of the characteristics to note in calculating incidence rate (incidence density) include:

• The participants can have different start times, and different end times.
• Some participants drop out of follow-up, some may die of other disease.

The above characteristics result in the following:

• Each participant could contribute different time at risk.
• We have incomplete (not uniform) follow-up on the cohort.
• The denominator = the sum of each person's time interval ("person time of follow-up").

Study Data Entry

We will enter the information for each participant individually. Let us start with specifying how many participants in the study, as well as the unit of time and the duration of the study.