The importance of basic statistical principles for the interpretation of epidemiological data

Statistical methods are playing an important and;increasing role in modern epidemiological research. We give a brief definition of some types of studies and the appropriate measures to describe the effect of risk factors under consideration. Based on a simple hypothetical example, we illustrate the influence of sample size, misclassification and measurement error, collapsing of categories, different approaches for dose response analysis, and adjustment for confounding factors on the assessment of a risk factor. Especially for the investigation of weak risk factors, the necessary decisions on the details of the analysis may have a serious impact on the final result. The issues have to be considered when judging the results and the value of a study and illustrate that wrong use of statistics can lead to false conclusions. The statistical aspects are illustrated by discussing a recently published study on the use of oral contraceptives as a potential risk factor for breast cancer.