# How to deal with variable new behavior in multiple linear regression

Let's suppose that I have one model with 6 dummy variables for the days of the week and from those in 5 days a specific event happen, suppose that is daytostudy variable and I want to predict how many exercises I can make in each day. At a specific moment in time (recent data) I've began to study also during Saturdays, so all my past behavior changes and the predicted value when I use daytostudy = 1 on Saturdays will have a new behavior.

What is the best way to deal with the change of the behavior of the new variable in a multiple linear regression model? Can you please point me also some references!?

"Interactions" allow you to relax the assumption that a variable has a constant relationship to the outcome. You can create an interaction by multiplying two variables (both of which must be included as "main effects" in your model).

For example, say that we wanted to predict age at first marriage (Y) using a sex dummy that equals 1 for males (X1) and another dummy that indicates the respondent has "traditional values" (X2).

Our model equation looks like: Y = B0 + B1*X1 + B2*X2

. . . and our goal is to estimate coefficients B0, B1 and B2.

But then if we hypothesize that although women tend to get married at a younger age than men, this difference is especially pronounced in "traditional families." (In your terms, "the behavior of the variable" changes).

You now want a model equation that looks like:

Y = B0 + B1*X1 + B2*X2 + B3*X1*X2

. . . and our new goal is to estimate coefficients B0, B1, B2, and B3.

Note that B3 will only affect the predicted value of Y if X1 and X2 = 1.

Our model now generates four separate predicted values.

                         Y = B0 +B1*X1 + B2*X2 + B3*X1*X2
= B0 +B1*0  + B2*0  + B3*0*1
= B0


                     Y = B0 + B1*X1 + B2*X2 + B3*X1*X2
= B0 + B1*0  + B2*1  + B3*0*1
= B0 + B2


                     Y = B0 + B1*X1 + B2*X2 + B3*X1*X2
= B0 + B1*1  + B2*0  + B3*1*0
= B0 + B1


                      Y = B0 + B1*X1 + B2*X2 + B3*X1*X2