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Re: post hoc power analysis of ttest
Posted:
Jan 22, 2013 10:35 AM


> This function doesn't help since it addresses only ttests of one normal > distributed data set and not comparing between two data sets using ttest2 > for instance...
Keren, the script below calculates the twosample ttest as a function of the size of the two samples and the magnitude of the difference between their means. It verifies by comparing the calculated result against the observed power in a simulation.
I hope this is what you need.
 Tom
delta = linspace(0,2,21); pwrcalc = zeros(size(delta)); pwrobs = zeros(size(delta)); for k=1:length(delta)
sigma = 1; d = delta(k) ; % difference in means as a multiple of sigma mu1 = 10; mu2 = mu1 + d*sigma; n1 = 20; n2 = 30; alpha = 0.05; crit = tinv(alpha/2, n1+n22); p = nctcdf([crit,crit], n1+n22, d/sqrt(1/n1+1/n2)); pwrcalc(k) = p(1) + 1p(2)
numout = 0; n = 10000; for j=1:n x1 = normrnd(mu1,sigma,n1,1); x2 = normrnd(mu2,sigma,n2,1); numout = numout + ttest2(x1,x2); end pwrobs(k) = numout/n end plot(delta,pwrcalc,'b', delta,pwrobs,'ro')



