> On Sep 18, 2016, at 12:39 PM, mviljamaa <mvilja...@kapsi.fi> wrote: > >> On Sep 18, 2016, at 11:01 AM, mviljamaa <mvilja...@kapsi.fi> wrote: >> Also if you, rather than doing what's done below, do: >> fit3 <- lm(kidmomhsage$kid_score ~ kidmomhsage$mom_age + kidmomhsage$mom_hs >> + kidmomhsage$mom_age * kidmomhsage$mom_hs) >> Then this gives the result: >> Call: >> lm(formula = kidmomhsage$kid_score ~ kidmomhsage$mom_age + >> kidmomhsage$mom_hs + >> kidmomhsage$mom_age * kidmomhsage$mom_hs) >> Coefficients: >> (Intercept) >> 110.542 >> kidmomhsage$mom_age >> -1.522 >> kidmomhsage$mom_hs >> -41.287 >> kidmomhsage$mom_age:kidmomhsage$mom_hs >> 2.391 >> Where the interaction term now seems properly interpretable. So perhaps this >> is the way to use interaction terms with lm. > > But why does > > fit3 <- lm(kidmomhsage$kid_score ~ kidmomhsage$mom_age * kidmomhsage$mom_hs) > > also give exactly the same result: > > Call: > lm(formula = kidmomhsage$kid_score ~ kidmomhsage$mom_age * kidmomhsage$mom_hs) > > Coefficients: > (Intercept) > 110.542 > kidmomhsage$mom_age > -1.522 > kidmomhsage$mom_hs > -41.287 > kidmomhsage$mom_age:kidmomhsage$mom_hs > 2.391 > > It's as if lm is interpreting there having to also be "independent" mom_age > and mom_hs variables, if there's just the interaction term. Why does it work > this way?
kidmomhsage$mom_age * kidmomhsage$mom_hs ... is expanded by the formula-engine so that it is exactly: kidmomhsage$mom_age + kidmomhsage$mom_hs + kidmomhsage$mom_age:kidmomhsage$mom_hs (That's essentially the definiton of the `*`-operator in the formula-world.) David Winsemius Alameda, CA, USA ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
