One-sample or two-sample t-test. This function is a wrapper for
svymean in the one-sample case and for
svyglm in the two-sample case. Degrees of freedom are
degf(design) for the one-sample test and degf(design)-1
for the two-sample case.
svyttest(formula, design, ...)
Formula, outcome~group for two-sample,
outcome~0 or outcome~1 for one-sample