The two commands you give below are certain to lead to very different results, because they are fitting very different models.
The first is a gaussian model for the response with a log link, and constant variance. The second is a gaussian model for a log-transformed response and identity link. On the original scale this model would imply a constant coefficient of variation and hence a variance proportional to the square of the mean, and not constant. Your problem is not particularly an R issue, but a difficulty with understanding generalized linear models (and hence generalized additive models, which are based on them). Bill Venables. ________________________________________ From: r-help-boun...@r-project.org [r-help-boun...@r-project.org] On Behalf Of pigpigmeow [gloryk...@hotmail.com] Sent: 19 June 2011 17:39 To: r-help@r-project.org Subject: [R] please help! what are the different using log-link function and log transformation? I'm new R-programming user, I need to use gam function. y <- gam(a ~ s(b), family = gaussian(link=log), data) y <- gam(log(a) ~ s(b), family = gaussian (link=identity), data) why [do] these two command [give different] results? I guess these two command results are same, but actally these two command results are different, Why? -- View this message in context: http://r.789695.n4.nabble.com/please-help-what-are-the-different-using-log-link-function-and-log-transformation-tp3608931p3608931.html Sent from the R help mailing list archive at Nabble.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. ______________________________________________ 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.