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The impact of regression towards the mean on probing changes in studies on the effect of periodontal therapy.

The purpose of this report was to evaluate the magnitude of the regression towards the mean (RTM) relative to the observed changes in probing measurements following periodontal therapy in 2 groups of patients. Regression lines were calculated for observed changes in probing depths and probing attachment levels related to the initial probing depths of the individual sites. Regression lines, adjusted for the RTM effect, were also calculated. For the extreme subgroups of recorded shallow and deep sites, the RTM effect (mm) was found to amount to a maximum of 0.6 mm. It appears unlikely that erroneous conclusions have been made in previous clinical studies evaluating probing depth changes in deep sites, since the RTM accounts for a limited portion of the observed changes. For probing depth changes in shallow sites and for probing attachment level changes in both shallow and deep sites, the RTM accounts for a larger proportion of the observed mm changes. It therefore seems prudent to be careful about interpretation of these changes in clinical studies, where the RTM effect has not been adjusted for. Whenever possible, duplicate initial recordings should be taken to allow determination of and subsequent adjustment for the RTM effect.