title: Evaluation of alternative statistical models for crossover studies to demonstrate caffeine adjuvancy in the treatment of tension headache.
abstract: This paper discusses alternative statistical models for the analysis of six crossover studies to determine whether better relief of tension headache occurs from treatment with an analgesic plus caffeine (C) than with the analgesic alone (A) or with placebo (P). Each patient in these crossover studies randomly received a pair of distinct medications in such a way as to treat the first two of four headaches with the initial medication in the pair and to treat the third and fourth headaches with the last medication in the pair. In order to have greater power for the C versus A comparison, three times as many patients were randomly assigned to the A:C and C:A sequence groups as to the A:P, C:P, P:A, and P:C sequence groups. An issue of statistical interest for these crossover studies is the extent to which the possibility of unequal carryover effects of the three medications influences the roles of alternative models for data analysis and the interpretation of results. When carryover effects for all three medications are equal, univariate analysis of variance for the difference scores between the average response for the first two headaches and the average response for the third and fourth headaches for each patient provides nearly the same power for pairwise treatment comparisons as more comprehensive multivariate methods for all four headaches. However, for comparisons concerning carryover effects and for treatment comparisons with adjustment for carryover effects, multivariate methods encompassing all four headaches jointly can provide greater power than univariate analysis for difference scores, particularly when there is low intraclass correlation for responses within the same patient. Another noteworthy role for multivariate methods in situations with potentially unequal carryover effects is their capacity to clarify whether multiple types of carryover effects occur across the second, third, and fourth headaches in the respective sequence groups. Multivariate models with alternative specifications of carryover effects are fit to the data from the six crossover studies to compare C, A, and P by weighted least squares. The role of potential variation among centers is addressed in these analyses by the use of stratified proportional means over centers, means of center means, and means ignoring centers. The primary focus of attention in the respective analyses is the evaluation of treatment comparisons with and without adjustment for potential differences among carryover effects of the treatments.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Biostatistics, School of Public Health, University of North Carolina, Chapel Hill 27599-7400.
authors.forename: G G
authors.initials: GG
authors.lastname: Koch
authors.forename: I A
authors.initials: IA
authors.lastname: Amara
authors.forename: J
authors.initials: J
authors.lastname: MacMillan
mesh_terms: D002110:Caffeine / Q000627:therapeutic use*; D018592:Cross-Over Studies; D004359:Drug Therapy, Combination; D006801:Humans; D015233:Models, Statistical; D015999:Multivariate Analysis; D012107:Research Design / Q000706:statistics & numerical data*; D018781:Tension-Type Headache / Q000188:drug therapy*
chemical_list: D002110:Caffeine