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?
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