Hi Elaine, If you want identical models, you need to use the same family and then the formula is the same. Here is an example with a built in dataset:
## these two are identical > coef(lm(mpg ~ hp + log(wt), data = mtcars)) (Intercept) hp log(wt) 38.86095585 -0.02808968 -13.06001270 > coef(glm(mpg ~ hp + log(wt), data = mtcars, family = gaussian)) (Intercept) hp log(wt) 38.86095585 -0.02808968 -13.06001270 ## not identical > coef(glm(mpg ~ hp + wt, data = mtcars, family = gaussian(link = "log"))) (Intercept) hp wt 3.88335638 -0.00173717 -0.20851238 I show the log link because the poisson family default to a log link, but that is equivalent to: log(E(y)) = Xb where X is your design matrix (intercept, A, B, log(C), log(D) for you). In short the link function operates on the outcome, not the predictors so even though the poisson family includes a log link, it will not yield the same results as a log transformation of two of your predictors. I do not have any online references off the top of my head, but it seems like you may be well served by reading some about generalized linear models and the concept of link functions. Cheers, Josh On Sun, Oct 28, 2012 at 8:01 PM, Elaine Kuo <elaine.kuo...@gmail.com> wrote: > > Hello list, > > I am running a regression using > > lm(Y~A+B+log(C)+log(D)) > > > Now, I would like to test if glm can produce similar results. > So the code was revised as > > glm(Y~A+B+C+D, family=poisson) (code 1) > > > However, I found some example using glm for lm. > It suggests that the code should be revised like > glm(Y~A+B+log(C)+log(D), family=poisson) (code 2) > > Please kindly advise which code is correct. > Thank you. > > Elaine > > [[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. -- Joshua Wiley Ph.D. Student, Health Psychology Programmer Analyst II, Statistical Consulting Group University of California, Los Angeles https://joshuawiley.com/ ______________________________________________ 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.
