title: Analysis of change--are base-line measurements needed? Some statistical comments on a common experimental design.
abstract: In experiments where the treatment effect is defined as the change of a variable during treatment, one may, under certain circumstances, obtain a higher precision by basing the statistical analysis on the after-treatment measurements alone instead of the change during treatment. The conditions for this are derived and an empirical illustration involving bacterial counts is presented. In the case of a cross-over trial it is found in this particular case that not using the base-line measurements reduces the number of patients needed by approximately 40%. In the case of a completely randomized design, about the same precision is obtained whether or not one utilizes the base-line values.
mesh_terms: D000328:Adult; D001419:Bacteria / Q000187:drug effects; D002710:Chlorhexidine / Q000008:administration & dosage / Q000494:pharmacology; D006801:Humans; D009067:Mouthwashes; D012107:Research Design; D012463:Saliva / Q000382:microbiology; D013223:Statistics as Topic
chemical_list: D009067:Mouthwashes; D002710:Chlorhexidine