# [R] How are interaction terms computed in lm's result / problems with interaction terms in lm?

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?
```
______________________________________________
R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
https://stat.ethz.ch/mailman/listinfo/r-help