Hi I have to calculate the sample size of a 2x2 crossover study. The primary efficacy endpoint is a continuous variable (FeNO) with a lognormal distribution, measured before (pre) and after (post) treatment. <1> Can I use proc power with paired t tests for Mean Ratio with Lognormal Data or is there a better way? proc power; pairedmeans test=ratio DIST=logNormal meanratio = cv = corr = npairs = . alpha = 0.05 power = 0.9; run; I have data from a pilot study very similar. The following procedure was used to analyze the efficacy endpoint. proc glm data=pilot; class seq drug period pat; model post_log = seq pat(seq) drug period pre_log/ ss3; *pre_log and post_log = before and after treatment suitably log trasformed. test h=seq e=pat(seq); lsmeans drug /pdiff stderr; run; <2> It is correct? <3> There is a proc glmpower corresponding to this ANCOVA analysis? To obtain the data for my proc power instead I should perform the following proc t test: proc ttest data=pilot dist=lognormal; var post1 post2 / crossover= (drug1 drug2); run; <4> meanratio = is geometric mean of treatment ratio with both sequence ? <5> cv = is geometric CV of treatment ratio with both sequence? <6> corr = how can I calculate it? If I submit the following procedure: proc ttest data=pilot dist=lognormal; var post_ratio_drug1_drug2; run; I get the same geometric mean but another CV. <7> Which CV should I use? But using only the post-treatment values I have to assume that values before treatment are homogeneous between treatment groups. <8> To avoid this how can I do? <9> I can analyze the post/pre ratio? <10> If it is distributed normally have to use a proc power with DIST = Normal? I hope not too 10 questions. Thanks in advance (Please use plain English) Favola
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