Effect of secular decline in caries on the evaluation of preventive dentistry demonstrations

Studies of dental preventive interventions often do not incorporate a concurrent, longitudinal control group in their study design. The existence of a secular decline in dental caries during the conduct of these studies may result in an overstatement of the effectiveness of the preventive intervention. Results of a recent large caries preventive study are analyzed using both a concurrent, longitudinal control group and an historical comparison group to demonstrate this effect. Differences in effectiveness of preventive procedures determined using the two types of control groups are sufficient to suggest a probable misinterpretation of results of studies using only historical comparisons.