expanding factor interactions within a formula

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I have many formulas (of class formula or Formula) of the form y ~ a*b, where a and b are factors.

I need to write a function that takes such a formula and returns a formula with all of the terms in the interaction "spelled out." Here is an example:

fac1 <- factor(c('a', 'a', 'b', 'b'))
fac2 <- factor(c('c', 'd', 'c', 'd'))
BigFormula(formula(x ~ fac1*fac2))

where BigFormula returns formula(x ~ a + b + c + d + a:c + a:d + b:c + b:d).

Is there a simple way to do this?

(The context: I am running many commands of the form anova(mod1, mod2), where mod2 nests in mod1, and where the right-hand side of both models contains terms like fac1*fac2. The point of these commands is to calculate F-statistics. The problem is that anova treats fac1*fac2 as three variables, even though it usually represents more than three variables. (In the code above, for example, fac1*fac2 represents eight variables.) As a result, anova underestimates the number of restrictions in the nested model, and it overestimates my degrees of freedom.)

r
5 Answers

I just stumbled across this solution:

  fac1 <- factor(c('a', 'a', 'b', 'b'))
  fac2 <- factor(c('c', 'd', 'c', 'd'))
  update(formula(x ~ fac1*fac2),~.)
user_76026150
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