Hello, Im trying to perform a post-hoc analysis to estimate the detectable differences (percentage difference of detectable

0 votes

Hello, Im trying to perform a post-hoc analysis to estimate the detectable differences (percentage difference of detectable

treatment means relative to control means) for a = 0.05 and a power of 80%. Does Web Power do this?...I have been

trying but cant find the correct way. Want to estimate the detectable differences of a t-test (two tailed).

If anyone can help me it would be great!!

My data are:

sample 1) N= 16; mean= 122.4; S.E ± 8.38

Sample 2) N = 20; mean= 111.4; S.E. ± 6.35

Thanks in advance!

Matías

+1 vote

The short answer is yes.

In the interface, if you leave the effect size field blank but supply the sample size, it will output the effect size to achieve the desired power. The effect size is also called the detectable difference or minimum detectalbe difference.

In the interface, if you leave the effect size field blank but supply the sample size, it will output the effect size to achieve the desired power. The effect size is also called the detectable difference or minimum detectalbe difference.

+1 vote

Let me provide a little more details here.

First, use the two-sample t-test power analysis, one can get the effect size is 0.967 with a power of 0.8 and the sample sizes \(n_1=16\) and \(n_2 = 20\).

Suppose the the common standard deviation of the two population can be estimated using the pooled standard deviation such that

\[ s_p = \sqrt{ \frac{(n_1-1)s_1^2+(n_2-1)s_2^2}{n_1+n_2-2} } = 7.32\]

assuming \(s_1=8.38\) and \(s_2=6.35\). Then the detected difference should be 0.967*7.32 = 7.07.