Also if you, rather than doing what's done below, do:

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`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.`

`However, in the above, is the coefficient 2.391 of`

`kidmomhsage$mom_age:kidmomhsage$mom_hs actually only that for mom_hs ==`

`1 in which case for mom_hs == 0 one would simply ignore the last`

`coefficient?`

`And would one still need to perform summations of kidmomhsage$mom_age`

`and kidmomhsage$mom_age:kidmomhsage$mom_hs coefficients, i.e. the`

`coefficient for kidmomhsage$mom_age = -1.522 + 2.391?`

On 2016-09-18 20:41, mviljamaa wrote:

I'm trying to use interaction terms in lm and for the following typesof 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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