This operator creates a proportion from two variables. It is
useful if your datasets includes vote variables coded as the number of voters,
instead of proportions. The NUM! operator works on two variables. Thus, if
rep88 is the number of voters in each district casting their ballots for the
Republicans and dem88 is the number voting for Democrats,
NUM!dem88&rep88 is the Democratic proportion of the two-party vote,
dem88/(dem88+rep88).
