It's the same as a normal t-test. You just have to keep in mind that you're testing against the null hypothesis H_{0}: μ_{D} = 0, where μ_{D} is the population mean of differences between pairs of observations. So it's
t_{0} = d(bar) / (s_{d} / √n)
where d(bar) and s_{d} are the sample mean and sample standard deviation for the differences between each pair.
edit: And just to be clear, n is the number of pairs of observations.
Basically, you treat this like a regular t-test problem, except your random variable is the difference between each pair of observations.
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