This is an R formula handling question. It arose in class. We were working on the Animals data in the MASS package. In order to see a relationship, you need to log brain and body weight. It's a fun one for teaching regression, if you did not try it yet. There are outliers too!
Students wanted to make a predicted value plot in the non-logged values of y, for comparison, and I wondered if I couldn't automate this somehow for them. It made me wonder how R manages formulae and if a transformation like log(y) can be be mechanically inverted. So we have something concrete to talk about, suppose x and y are variables in dat, a person fits m1 <- lm(log(y) ~ log(x), data = dat) termplot shows log(y) on the vertical. What if I want y on the vertical? Similarly, predict gives values on the log(y) scale, there's no argument like type = "untransformed". I want my solution to be a bit general, so that it would give back predicted y for formulae like sqrt(y) or exp(y) or log(y + d) or whatever other math people might throw in there. Here's what I can tell so far about R's insides. The formula handler makes a list out of the formula, I can get that from the terms object that the model generates. The formula list has "~" as element 1, and "log(x)" becomes element [[2]]. Where in the R source code can I see how R "looks at" the symbol log(y) and discerns that there is a variable y that needs to be logged? If I could understand that, and if R has a table of inverse functions, then maybe I could see what to do. If you have ideas, I'm very grateful if you share them. pj -- Paul E. Johnson Professor, Political Science Assoc. Director 1541 Lilac Lane, Room 504 Center for Research Methods University of Kansas University of Kansas http://pj.freefaculty.org http://quant.ku.edu [[alternative HTML version deleted]] ______________________________________________ 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.
