This paper describes the methodological aspects of calculation of incidence rates from incomplete datain occupational epidemiology. Proportionate measures in epidemiological studies are useful e.g. to describethe proportion of slips, trips and falls compared to other types of injury mechanisms within singleage-strata. However, a comparison of proportions of slips, trips and falls among the different age-stratagives no meaning and can hamper the conclusions. Examples of a constructed example and some selectedstudies show how estimates of incidence rates can be calculated from the proportionate data by applyingestimates of denominators available from other information. The calculated examples show how the risksbased on the incidence rates in some cases differ from the risks based on the proportionate rates withthe consequence of hampering the conclusions and the recommendations for prevention. In some casesthe proportionate rates give good estimates of the incidence rates, but in other studies this might causeerrors. It is recommended that estimates of the incidence rates should be used, where this is possible, byestimation of the size of the population. The paper is intended to be useful for students and teachers inepidemiology by using the attached Excel training file.