jim holtman wrote: > ?cor.test > > and the help page says: > > formula: a formula of the form ~ u + v, where each of u and v are > numeric variables giving the data values for one sample. The samples > must be of the same length. > Yes, but does that answer the question? Seems to me that Joe knows what it does, just not why.
The logic behind this sort of model formulas is slightly warped, but has been with us at least since S-PLUS's crosstabs(). R's xtabs() is quite similar. The basic idea is that you can describe a contingency table with n dimensions using count ~ f1 + f2 + .... + fn and leave the count off if it would be the constant 1. The thing that I, at least, find a little warped is that "+" in model formulas usually prescribes an additive structure, which isn't really the case here. The ":" operator was arguably a better choice, but, well, another choice was made, and has stuck. The final step is to say that correlation between two quantitative variables is similar enough to RxC tables for categorical variables to let you use the same syntax. ~x+y does have the advantage of not implying a direction as y~x or x~y would do. > On 1/12/08, Joe Trubisz <[EMAIL PROTECTED]> wrote: > >> Hi... >> >> I'm trying to understand the following syntax: >> >> cor.test(~mortality + hardness,data=water,method="pearson") >> >> which is the same as: >> >> cor.test(water$mortality,water$hardness,data=water,method="pearson") >> >> Can anyone point me to the correct doc or explain to me how to >> interpret "~mortality + hardness"? >> >> Thanks, >> Joe >> >> ______________________________________________ >> R-help@r-project.org mailing list >> 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. >> >> > > > -- 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 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ R-help@r-project.org mailing list 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.
