**Cohort studies measuring incidence rates**

The computation of effects with incidence rates is similar to calculation of effects from incidence proportions (risk). The incidence rate of disease in exposed (IRe) and unexposed (IRu) can be computed as follows:

A rate difference can be computed:

The relative effect of the exposure on disease occurrence can be measured by computing the rate ratio minus 1.

The rate ratio is:

**Example **

*Breast cancer cases and person-years of observation for women with tuberculosis repeatedly exposed to multiple x-ray fluoroscopies and unexposed women with tuberculosis*

Radiation exposure | Person-years | Breast cancer | Rate/10000 p-y | Rate ratio | Rate difference | Relative effect |

Yes | 28010 | 14 | 14.6 | 1.86 | 6.7 | 0.86 |

NO | 19017 | 15 | 7.9 |

Source: Boice & Monson [1]

One can express the result by saying that the relative effect is 0.86 which would suggest an 86 % increased rate of breast cancer among exposed. One can also express the results by saying that the rate of breast cancer is 1.86 times higher in the exposed cohort than in the unexposed cohort.