December 2006

Objectives: To investigate the relationship between incidence and prevalence of caries. The principles are illustrated by means of Lexis diagrams, which show the relationship between cases of caries belonging to a population with a certain incidence and natural course of the disease in time. The analysis shows how studies on prevalence yield associations that reflect the determinants of the disease progression just as much as the causes of disease. Also the diagnostic criteria of the disease affect the prevalence estimate. The presence of fluoride changes the course of the disease and the prevalence estimate even with an unchanged incidence rate. This knowledge about incidence and prevalence should be taken into consideration when data from cross-sectional and time-series studies are interpreted. In an example from Norway the prevalence of caries was reduced. During the same period fluoride was made universally available, invasive treatment criteria were changed to less invasive and the time between the regular examinations were extended. These factors affect the estimates of caries made by cross-sectional recordings. Key words: Cross-sectional estimates, fluoride, incidence, prevalence, treatment criteria