t-test (sample-1 sample-2 &optional (tails both) (h0mean 0))
Returns the t-statistic for the difference in the means of two samples, which
should each be a sequence of numbers. Let D=mean1-mean2. The null hypothesis
is that D=0. The alternative hypothesis is specified by tails': :both' means
D/=0, :positive' means D>0, and :negative' means D<0. Unless you're using
:both tails, be careful what order the two samples are in: it matters!
The function also returns the significance, the standard error, and the degrees
of freedom. Signals standard-error-is-zero' if that condition occurs. Signals<br> insufficient-data' unless there are at least two elements in each sample.