[R] LM: Least Squares on Large Datasets OR why lm() is designed the w ay it is
Vadim Ogranovich
vograno at arbitrade.com
Fri Aug 9 19:56:29 CEST 2002
Hi,
I have always been wondering why S-Plus/R can not fit a linear model to an
arbitrary large data set given that, I thought, it should be pretty
straightforward. Sometime ago I came across a reference to LM package,
http://www.econ.uiuc.edu/~anovo/LM.html, by Roger Koenker and Alvaro Novo.
So I thought here it is at last, but to my surprise this project hasn't made
to the recommended packages and its development seems to be stopped. I take
it as a strong evidence that there is a conceptual problem in doing this
sort of things and I thought it would be very educational for me to
understand it.
Here is how I would structure lm object, please feel free to point mistakes
out.
Suppose we want to analyze lm(Y ~ X), where Y is a vector and X is a matrix
1. Under the classical assumptions of normality and independence of the
residuals all information about the model is encapsulated in the covariance
matrix of [Y,X] and the observation count, i.e. length(Y). These include
variance of coefficients, their significance levels, ability to compute
predictions, etc. Moreover, all sub-models, i.e. a regression on any subset
of X columns are also readily computable, as well as ANOVA.
Given this I'd store the covmatrix of [Y,X] and the count on an lm object
and write summary.lm, anova.lm, step, stepAIC functions in terms of these
two members only.
I guess this is the idea behind the LM package.
2. There is whole lot of tests that are designed to check the classical
assumptions of normality of the residuals, detect influential points, etc.
Obviously these can not possibly be carried out without the residuals, etc.
So the lm object should provide a slot for the residuals, but whether the
residuals are in fact computed should not affect the functions mentioned in
the previous paragraph.
I will appreciate any comment on this "design".
Thanks, Vadim
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