Validity issues in split-mouth trials

• Published in: Journal of clinical periodontology Vol. 19; no. 9 Pt 1; p. 625
• Main Authors: Hujoel, P P, DeRouen, T A
• Format: Journal Article
• Language: English
• Published: United States 01.10.1992
• Subjects: Humans; Models, Statistical; Periodontal Diseases - therapy; Reproducibility of Results; Research Design; Treatment Outcome
• ISSN: 0303-6979
• DOI: 10.1111/j.1600-051X.1992.tb01709.x

The main purpose of the split-mouth design is to remove all components related to differences between subjects from the treatment comparisons. By making within-patient comparisons, rather than between-patient comparisons, the error variance (noise) of the experiment can be reduced, thereby obtaining a more powerful statistical test. Unfortunately, comparisons made on a within-patient basis have potential disadvantages. Treatments may have effects on experimental units other than those which they were assigned to (carry-across effects). Such effects cannot be estimated from split-mouth data. Neither can treatment effects be estimated. The estimable parameter in a split-mouth design is the treatment effect plus the sum of all carry-across effects. Unless a priori knowledge indicates that no carry-across effects exist, reported estimates of treatment efficacy are potentially biased. In the design of split-mouth clinical trials, potential gain in precision should be carefully weighed against a potential decrease in validity.