a formula, a terms object, or a design.
If a design, then data need not be given, and the formula "~ ." is implied.
OPTIONAL ARGUMENTS:
data:
a design or data frame.
If data is not given, the search list is used to find the objects in
formula.
Usually a data frame should be attached when data is not given.
na.action:
function that specifies how missing values are handled.
VALUE:
a vector or a list whose length is the number of terms in formula.
If all of the levels in each of the terms are replicated the same number
of times, then a vector is returned that contains the number of replicates
for each term.
Otherwise, a list is returned that contains the information on replication,
one component for each term;
an unbalanced main effect will be a vector, an unbalanced two factor
interaction will be a matrix, and so on.
DETAILS:
A test for balance is (!is.list(replications(formula,data))).