# How to perform paired sample t-test when the standard error of the difference is zero?

I ran a paired T-test to determine the mean difference between pre and post intervention of one of the variables in my study. However the test was not run as the SPSS output showed "the correlation and t cannot be computed because the standard error of the difference is 0".

What should I do to get the p-value of the mean difference for that variable?

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Is the value of the paired difference exactly the same for every pair? That is what "the standard error of the difference is 0" seems to mean. –  Macro Jun 24 '12 at 3:44
I think the difference between your two columns is a constant. Constant has no variability and thus no test can be done. –  ttnphns Jun 24 '12 at 3:50
Of course a test can be done, @ttn: just not a t test! –  whuber Jun 24 '12 at 14:12
Please add some information on what kind of data you have... do you meet the assumptions of the t-test? –  John Jun 24 '12 at 14:23
Sorry, @whuber, I meant "no t test..." or "the t-test can't..." –  ttnphns Jun 24 '12 at 16:00
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