Does your SAP restrict your response to % change in counts? If not, I would strongly consider looking at counts as the dependent variable, and modeling an appropriate distribution (poisson, negative binomial, and possibly zero inflated versions of these). At the FDA/Industry Statistics Workshop held recently in Washington, DC, Dr. Stephen Senn led a short course on statistical issues in drug development. One slide really struck home (and is applicable here): Seven Deadly Sins in Measuring Clinical Trial Outcomes Using baslines to construct change-scores Percentage change from baseline Crude corrections in general Correcting for post-randomsation covariates Ignoring titration Treating ordinal data as categorical Creating dichotomis from continuous data It seems to me that this hits the first two. Ratios generally do not have good distributional properties. He suggested analysis of covariance with log(y) as the dependent variable and log(x), i.e., the log of the baseline value as a covariate. This should get around the negative percentages, but you may be locked in by your SAP. Good luck, Steve Denham
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