Clinical prediction rules should help the medical doctors to increase the accuracy of diagnostic and prognostic assessments in situations where decision making is complex. A decision rule is a clinical tool that quantifies the contributions that various components of the history, physical examination, laboratory results and imaging techniques make toward the diagnosis or prognosis in an individual patient. Before decision rules are broadly used in a clinical situation they have to be validated. This can be done by using part of the original data to perform a validation. To answer the question about the generalizability of the rules they have to be tested in an additional, prospectively collected data set.