Use this feature to specify a
variable
that defines different strata of your experiment. In a stratified analysis, categorical variables can be adjusted for and designs such as matched or nested case-control can be accommodated by creating separate

contingency tables
for each stratum (as defined by the

strata variables
) and combining each stratum's

test statistic
into a single statistic without compromising degrees of freedom.

Cochran-Mantel-Haenszel
statistics are used to test for

association
controlling for these strata.