Optionalby?: stringGrouping column with exactly two levels → two-sample test, difference = first level − second (sorted).
OptionalequalVar?: booleanTwo-sample only: pool variances instead of Welch.
Optionalpaired?: stringSecond column of the same length → paired test on column − paired.
Column-based t-test: one-sample by default, two-sample with
by(a 2-level column), paired withpaired.