t-test weird degrees of freedom

Hi,

I did some t-test (two-tailed) and keeps geting weird degrees of freedom.

I have some pairs of vectors, all of size = [1 144], so n=144 observations for each group

ttest gave me the expected df (142) for some tests, but I also got 139 in another one, and even... 124 in an other one.

What is going on ? Why is the degree of freedom not 142 in all those tests ?

Thank you

[Fatemeh Shomal Zadeh]

Hi,

Sorry, can you help me and explain how I can get DoF (degrees of freedom) from my test?

Thanks

Answer 1

[Tom Lane]

I can think of two possibilities. First, if you have any NaN values, they are treated as missing and the df would be reduced accordingly. Second, if you ask for a test based on 'unequal' variances, the df is computed using a Satterthwaite approximation and may be a non-integer.

[laurie]

thanks ! it was a bit of both actually !

[Fatemeh Shomal Zadeh]

Hi,

Sorry, can you help me and explain how I can get DoF (degrees of freedom) from my test?

Thanks