5
votes
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I am trying to calculate multiple regression in R without intercept.

My data is as follow:

    y  <- c(60.323,61.122,60.171,61.187,63.221,63.639,64.989,63.761,66.019,67.857,68.169,66.513,68.655,69.564,69.331,70.551)
    x1 <- c(83,88.5,88.2,89.5,96.2,98.1,99,100,101.2,104.6,108.4,110.8,112.6,114.2,115.7,116.9)
    x2 <- c(107.608,108.632,109.773,110.929,112.075,113.27,115.094,116.219,117.388,118.734,120.445,121.95,123.366,125.368,127.852,130.081)

In this case, (I believe?) I am getting the coefficients WITH intercept:

lm(formula = y ~ x1 + x2)

I would like to get the coefficients WITHOUT intercept. I tried this:

lm(formula = y ~ x1 + x2 -1)

Is this correct? If so, my question would be: How can I calculate WITHOUT intercept without changing the x values (on the right side of the tilde), but by changing something on the y value (on the left side of the tilde). For instance:

lm(formula = y -1 ~ x1 + x2)

Gets a different (and presumably incorrect coefficient estimation).

I know your question is ... why do you have to only change the y values? The reason is because I am writing code in C to do this, and I do not want to change the dimensions of X by adding a -1 at the end because that would require dynamic array allocation, which is very meticulous for me.

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5
  • $\begingroup$ You can't. Including - 1 on the LHS just subtracts 1 from the DV. Coefficients are only estimated for the RHS. I'm not sure I follow your rationale. $\endgroup$ – Roland Mar 24 '15 at 13:04
  • $\begingroup$ I am not sure I follow you rationale either. Anyway... this may be of help: model.matrix() returns the design matrix of your model object. $\endgroup$ – ocram Mar 24 '15 at 13:05
  • $\begingroup$ This is documented in ?lm. In order to remove intercept you can do either y ~ x - 1 or y ~ 0 + x so lm(formula = y ~ x1 + x2 -1) or similarly lm(formula = y ~ 0 + x1 + x2) is the way to go. $\endgroup$ – David Arenburg Mar 24 '15 at 13:10
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    $\begingroup$ @DavidArenburg Please read my comment again and more carefully. I'm not wrong. $\endgroup$ – Roland Mar 24 '15 at 13:12
  • $\begingroup$ @Roland, right, you meant to the second option mentioned by the OP. $\endgroup$ – David Arenburg Mar 24 '15 at 13:14
10
votes
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The formula

lm(formula = y ~ x1 + x2)

will include an intercept by default.

The formula

lm(formula = y ~ x1 + x2 -1)

or 

lm(formula = y ~ x1 + x2 +0)

is how R estimates an OLS model without an intercept. The formula

lm(formula = y-1 ~ x1 + x2)

estimates a model against a dependent variable y with 1 subtracted from it.

Centering all terms at their mean will also enforce a zero intercept.

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