A re-examination of within-mouth correlations of attachment level and of change in attachment level.

Using a data set from the Forsyth Dental Center consisting of repeated measurements of periodontal attachment level on 22 patients over 1 year, we performed correlation analyses at 4 time points (baseline, 1 month, 6 months and 9 months) for 3 units of measurement (sites, teeth and quadrants). Values of the intraclass correlation coefficient were obtained for the cross-sectional measurements and for changes from baseline. The values were positive and often of appreciable magnitude, indicating that measurements of attachment level and of change in attachment level within the same mouth are positively correlated. Taking sites rather than patients as the units of statistical analysis in comparative clinical trials in periodontology will therefore tend to produce underestimated standard errors and overstated statistical significance.