Cross-tabulation
crosstab
uses grp2idx
to assign a positive integer to each distinct value. tbl(i,j)
is a count of indices where grp2idx(x1)
is i
and grp2idx(x2)
is j
. The numerical order of grp2idx(x1)
and grp2idx(x2)
order rows and columns of tbl
, respectively.
In this case, the returned value of tbl(i,j,...,n)
is a count of indices where grp2idx(x1)
is i
, grp2idx(x2)
is j
, grp2idx(x3)
is k
, and so on.
crosstab
computes the p-value of the chi-square test statistic using a formula that is asymptotically valid for a large sample size. The approximation is less accurate for small samples or samples with uneven marginal distributions. If your sample includes only two variables and each has two levels, you can use fishertest
instead. This function performs Fisher’s exact test, which does not depend on large-sample distribution assumptions.