t-test calculation correct?
As this is apparently a post hoc test, this is wrong. The results are biased. You have provided a nice example of how to do irreproducible science. Consult a local statistician for what this means if you do not know. -- Bert Gunter
On Mon, Jan 17, 2011 at 4:35 AM, Sascha Vieweg <saschaview at gmail.com> wrote:
A multinomial logit model (N=192) revealed (besides others) the following statistics for the outcome, y, and one predictor, x: - y = A (baseline, n=34) - y = B (n=26), B(x)=0.7323 (SE=0.2384) - y = C (n=132), B(x)=0.6535 (SE=0.2041) With a t-test I want to explore whether the two predictors differ significantly, and I use the following calculation (according to Bortz, 2005, p.140): ########## dm <- 0.7323 - 0.6535 se.dm <- sqrt( (0.2384 / (34 + 26)) + (0.2041 / (34 + 132)) ) t.val <- dm / se.dm pval <- (1 - pt(t.val, df=(34+26+132) )) * 2 ########## My question is where this calculation is wrong and why. Ref.: Bortz, J. (2005). Statistik f?r Human- und Sozialwissenschaftler (6. Aufl). Berlin: Springer. -- Sascha Vieweg, saschaview at gmail.com
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Bert Gunter Genentech Nonclinical Biostatistics 467-7374 http://devo.gene.com/groups/devo/depts/ncb/home.shtml