EVALUATING THE BURST HYPOTHESIS AT A SITE-SPECIFIC LEVEL USING THE LACK-OF-FIT TEST

IT HAS BEEN HYPOTHESIZED THAT periodontal disease progresses by means of sudden losses of periodontal attachment surface area. Obtaining rel iable tests of this burst hypothesis has proven to be difficult; the s ignal (true model of disease progression) often gets lost in the noise . The purpose of this study was to determine how reliably we could dis tinguish sudden changes from linear disease progression at a site usin g a time series of clinical attachment levels. Specifically, the follo wing question was investigated: If, in reality, disease progresses by means of sudden changes in clinical attachment level (bursts), and a l inear model is fitted to these data, what is the likelihood of rejecti ng the linear model using the lack-of-fit test? This likelihood was de termined as a function of the probing measurement error (range: 0.2 to 1.0 mm) and the number of clinical examinations over time. The result s suggested that bursts of 2 mm or smaller cannot be reliably distingu ished from linear disease progression using the lack-of-fit test, exce pt under unusual clinical circumstances. Under typical clinical circum stances, burst sizes needed to be 3 to 5 mm in order to be reliably di stinguished from linear disease progression. These results are probabl y overly optimistic. The ability to verify the burst hypothesis at the site level is likely to be even less than our results indicate becaus e of various assumptions that were required. We conclude that the lack -of-fit test will reliably reject the linear model at a site-specific level only if true disease progresses in such a fashion that a handful of sudden changes leads to a tooth mortality event.