I'm having difficulties to find the right model formula for my model:

$Y_i=a+bX_i$ where Y and X are both deflated by another variable

y1/def ~ x1/def + x2/def 

returns a model with interactions. How can i prevent R from doing so?

  • $\begingroup$ Did this help you get the desired behavior? $\endgroup$
    – Steve S
    Commented Jul 29, 2014 at 6:48
  • $\begingroup$ As a word of caution, be careful with dividing by def, however--if it's a random variable then dividing both sides by def could completely change the functional form of your regression. Just throwing that out there... $\endgroup$
    – Steve S
    Commented Jul 29, 2014 at 7:36
  • $\begingroup$ Thank you for your answer and the hints on deflating. I wasnt able to check yet. Will comment when I did so. $\endgroup$
    – Gritti
    Commented Jul 29, 2014 at 14:52

1 Answer 1


If you want to perform a transformation on the variables in the regression, you'll have to use the I() function:

I(y1/def) ~ I(x1/def) + I(x2/def)

Try that instead and see if it works the way you want it to...


Basically, you use the I() function in a formula whenever you want an expression to be treated "as is". For example, if you have a data frame with three columns, a, b, & c, and you want to regress c onto the sum of columns a and b you would write: c ~ I(a + b) since--as you've seen yourself--entering: c ~ a + b would give you a totally different regression.


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