lm() and interactions in model formula for x passed as matrix
Hi Bill, If you can put all (and only) your variables into a dataframe, (for example: X <- data.frame(y, x1, x2, x3) ) then another alternative to David's solution would be: lm(y ~ .^3, data = X) '.' will expand to every column except y, and then the ^3 will get you up to 3-way interactions. Cheers, Josh On Sun, Dec 5, 2010 at 12:19 PM, William Simpson
<william.a.simpson at gmail.com> wrote:
Suppose I have x variables x1, x2, x3 (however in general I don't know how many x variables there are). I can do X<-cbind(x1,x2,x3) lm(y ~ X) This fits the no-interaction model with b0, b1, b2, b3. How can I get lm() to fit the model that includes interactions when I pass X to lm()? For my example, lm(y~x1*x2*x3) I am looking for something along the lines of lm(y~X ...) where ... is some extra stuff I need to fill in. Thanks for any help. Bill
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Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.com/