boxcox from MASS and bct from TeachingDemos do different things. The boxcox function does not return the transformed y values, it returns log-likelihood values for various values of lambda, these values can be used to decide which value of lambda to use (generally it is used by giving a sequence of lambda values then looking at the plot (see the plotit argument)).
It is generally not a good idea to just take the lambda with the highest log-likelihood, rather look for values of lambda within the confidence interval that make scientific sense. Once you have decided on a value of lambda to use then you can use the bct function from TeachingDemos (or other ways) to compute the transformed y values. Hope this helps, -----Original Message----- From: [EMAIL PROTECTED] on behalf of Torsten Mathies Sent: Sat 7/29/2006 12:52 AM To: [email protected] Subject: [R] boxcox transformation I've got a vector of data (hours to drive from a to b) y. After a qqplot I know, that they don't fit the normal probability. I would like to transform these data with the boxcox transformation (MASS), that they fit the model. When I try ybx<-boxcox(y~1,0) qqnorm(ybx) the plot is different from library (TeachingDemos) ybct<-bct(y,0) // qqnorm(ybct) How can I transform y to fit with the normal probability model? Yours Torsten ______________________________________________ [email protected] 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. [[alternative HTML version deleted]] ______________________________________________ [email protected] 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.
