Transform an Allowable Formula for 'loglm' into one for 'terms'
- A formula conforming to the conventions of
loglm, that is, it may allow dimension numbers to stand in for names when specifying a log-linear model.
The model fitting function
loglm fits log-linear models to
frequency data using iterative proportional scaling. To specify
the model the user must nominate the margins in the data that
remain fixed under the log-linear model. It is convenient to
allow the user to use dimension numbers, 1, 2, 3, ...for the
first, second, third, ..., margins in a similar way to variable
names. As the model formula has to be parsed by
1 in a special way and requires parseable variable names,
these formulae have to be modified by giving genuine names for
these margin, or dimension numbers.
denumerate replaces these
numbers with names of a special form, namely
n is replaced by
.vn. This allows
terms to parse the formula in the usual way.
- A linear model formula like that presented, except that where
dimension numbers, say
n, have been used to specify fixed margins these are replaced by names of the form
.vnwhich may be processed by
denumerate(~(1+2+3)^3 + a/b) ~ (.v1 + .v2 + .v3)^3 + a/b