I'm trying to use interaction terms in lm and for the following types of
models:
fit3_hs <- lm(kidmomhsage$kid_score ~ kidmomhsage$mom_age +
kidmomhsage$mom_hs + kidmomhsage$mom_age * 1)
fit3_nohs <- lm(kidmomhsage$kid_score ~ kidmomhsage$mom_age +
kidmomhsage$mom_hs + kidmomhsage$mom_age * 0)
where you see the last term being the interaction term (it's
mom_age*mom_hs where mom_hs takes values 0 or 1), the results are
causing a bit of confusion.
fit3_hs returns:
Call:
lm(formula = kidmomhsage$kid_score ~ kidmomhsage$mom_age +
kidmomhsage$mom_hs +
kidmomhsage$mom_age * 1)
Coefficients:
(Intercept) kidmomhsage$mom_age
70.4787 0.3261
kidmomhsage$mom_hs
11.3112
fit3_nohs returns:
Call:
lm(formula = kidmomhsage$kid_score ~ kidmomhsage$mom_age +
kidmomhsage$mom_hs +
kidmomhsage$mom_age * 0)
Coefficients:
kidmomhsage$mom_age kidmomhsage$mom_hs
3.368 11.568
Now why is (Intercept) term missing from the second one?
Also since in the first one the interaction term's coefficient should be
added to the coefficient of mom_age, then is the return value of
kidmomhsage$mom_age 0.3261 the sum of the coefficient of mom_age and the
coefficient of the interaction term? Or would I need to produce the sum
myself somehow?
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