Probability computations are used to characterize new periodontal diagnostic tests and are an integral facet of risk assessment for periodontal diseases. However, misinterpretation of these data can result in confusion and erroneous conclusions regarding the relationship of test results to the presence or absence of disease in a subject, or to episodes of periodontal destruction at specific sites. This paper was written to provide clinicians with a primer to help in understanding calculations used to evaluate diagnostic tests and risk assessment.