rkevinbur...@charter.net wrote:
I am just curious. Every once and a while I see an attribute attached to an object called
"assign". What meaning does this have? For example:
dist ~ speed, data=cars
forms a matrix like:
num [1:50, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
- attr(*, "dimnames")=List of 2
..$ : chr [1:50] "1" "2" "3" "4" ...
..$ : chr [1:2] "(Intercept)" "speed"
- attr(*, "assign")= int [1:2] 0 1
The "dimnames" attribute is fairly self-explanatory. I just am not sure what the
"assign" attribute means.
It has to do with the mapping between terms of the formula and columns
of the design matrix:
> str(model.matrix(Ozone~factor(Month)+Wind,data=airquality))
num [1:116, 1:6] 1 1 1 1 1 1 1 1 1 1 ...
- attr(*, "dimnames")=List of 2
..$ : chr [1:116] "1" "2" "3" "4" ...
..$ : chr [1:6] "(Intercept)" "factor(Month)6" "factor(Month)7"
"factor(Month)8" ...
- attr(*, "assign")= int [1:6] 0 1 1 1 1 2
- attr(*, "contrasts")=List of 1
..$ factor(Month): chr "contr.treatment"
I.e. columns 2:5 belong to the first non-intercept term, factor(Month).
(Notice that it is implicitly assumed that you have the corresponding
terms() output to hand, likewise for the "contrasts" attribute.)
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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