From: Rolf Turner <r.turner_at_auckland.ac.nz>

Date: Tue, 04 Nov 2008 09:15:14 +1300

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon 03 Nov 2008 - 20:18:31 GMT

Date: Tue, 04 Nov 2008 09:15:14 +1300

On 4/11/2008, at 4:30 AM, J. Sebastian Tello wrote:

> Does anyone know of a literature reference, or a piece of code that

*> can help me
**> calculate the amount of variation explained (R2 value), in a
**> regression constrained
**> to have a slope of 1 and an intercept of 0?
*

The question is ``wrong''. The idea of ``amount of variation
explained''

depends on decomposing the ``total sum of squares'' into two pieces
--- the sum of

squares of the residuals what is left over which is the sum of
squares ``explained

by the model''. In the usual regression setting this is

sum((y_i - ybar)^2) = sum((y_i - yhat_i)^2) + sum((yhat_i - ybar)^2)

or

SST = SSE + SSR (T for total, E for error, R for regression)

where yhat_i results from fitting the model by least squares.

The R-squared value is SSR/SST or 1 - SSE/SST. (Or this quantity time 100%.)

However if you constrain the slope to be 1 and the intercept to be 0
then

yhat_i = x_i and the forgoing identity does not hold. The problem is
that

the ``sum of squares left over'' can be negative (and hence not a sum
of squares).

I.e. in this case you have

SST = SSE + something

where ``something'' is not necessarily a sum of squares.

Thus you can have the ``amount of variation explained'' being negative!

E.g. x_1 = -1, x_2 = 1, y_1 = 1, y_2 = -1. In this setting the ``total sum of squares'' is 2 and the ``residual sum of squares'' is 4, so the ``amount of variation explained by the model'' is -2, or you could say that R-squared is -100%. (!!!)

Bottom line --- the R-squared concept makes no sense in this context.

The R-squared concept is at best dubious, and should be used, if at all, only in the completely orthodox setting.

cheers,

Rolf Turner

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