The FREQ statement specifies one numeric variable that tells how many clustering observations belong to the cluster. If the
FREQ statement is omitted, PROC TREE uses the variable `_FREQ_`

to specify the number of observations per cluster. If neither the FREQ statement nor the `_FREQ_`

variable is present, each leaf is assumed to represent one clustering observation, and the frequency for each internal node
is found by summing the frequencies of its children.