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

`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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