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 `insufficient-data' unless there are at least two elements in each sample.