INCREASING NUMBER OF TEETH PRESENT IN A QUASI-LONGITUDINAL STUDY IN ADULTS - A METHODOLOGICAL NOTE

This paper discusses the impossible result of an increase in the numbe r of teeth present in a quasi-longitudinal study of an adult populatio n. In 1983 and 1994, random samples were drawn from birth cohorts 1939 -48 and 1930-38. The paradoxical result that adults acquired new teeth in the quasi-longitudinal study gave rise to questions about the comp arability of the two samples. Four main sources of failure in comparab ility are described and discussed: instrumental stability, criteria co nsistency, response bias, and compositional change in aging cohorts. T he lower response rate among persons with two edentulous jaws, and pos sibly among persons with one dentate jaw, was concluded to be the main reason for the increase in the overall group median and mean number o f teeth present. In general, in studies with a similar study design, p roblems of comparability between the samples demand one's attention an d should therefore be carefully evaluated. Compositional change of the population as a source of failure in sample comparability cannot be a voided, while the other three sources (instrumental stability, criteri a consistency, response bias) can largely be avoided by careful planni ng and efficient use of resources.