MATLAB: T-test weird degrees of freedom

degrees of freedomdfttest

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

Best Answer

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.

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Wayne

P.S. On MATLAB Answers you are supposed to accept the answer if you have had your question answered so other people know that they don't have to answer it.

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If you have a univariate data then you can test it for normality using Anderson-Daarling test [1] or Kolmogorv-Smirnov test [2].

[1] http://www.mathworks.com/matlabcentral/fileexchange/14807-andartest ... you will have to modify the code slightly to get the p-value

[2] implemented as 'kstest' function in Statistics Toolbox

Unfortunately, I am not familiar with other tests to see if the data was drawn from other types of distributions ...

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