[Gretl-users] variance in mean test

Well, as you can see from my recent posts I have been using the test statistic calculator for the last couple of days. Here's another question about it: When testing the difference of two means (Utilities -> test statistic calculator), I cannot reproduce the standard deviation used in the denominator. For example, using mu1=1, sigma1=2, n1=25, mu2=1, sigma2=3, n2=36 gretl's test statistic output is 0/0.687129. What's the formula for that value? I would use \sqrt(4/25 + 9/36) = 0.6403... in the denominator instead, and I couldn't get gretl's ouput by some variations. Thanks for your help, Sven