A comparison of increment and incidence density analyses in evaluating the anticaries effects of two dentifrices

This study aimed to determine whether incidence density (ID) calculations of caries incidence rates would provide a more sensitive means of detecting caries-preventive effects than would traditional techniques. A secondary analysis was conducted using data from a 1981 study in which three dentifrices were compared in a double-blind randomized clinical trial. Subjects were examined at baseline and 1, 2 and 3 years after baseline. Three-year DMFS increments were calculated for 1,754 subjects attending the baseline and 3-year examinations. Caries ID rates then were calculated for 2, 661 subjects who had at least two examinations, using each surface's net increment (-1, 0 or +1) as the numerator and the surface's time at risk as the denominator. Despite theoretical advantages, the ID method did not alter the conclusions drawn using DMFS increments, apparently because (a) subjects lost to follow-up were similar to those completing the study, and (b) loss to follow-up was similar among treatment groups.