on 09/09/2008 09:59 AM Williams, Robin wrote: > Hi, > Please could someone explain how this element of predict.lm works? >>From the help file > ` > newdata > An optional data frame in which to look for variables with which to > predict. If omitted, the fitted values are used. > ' > Does this dataframe (newdata) need to have the same variable names as > was used in the original data frame used to fit the model?
Yes. Also, see the Note in ?predict.lm: "Variables are first looked for in newdata and then searched for in the usual way (which will include the environment of the formula used in the fit). A warning will be given if the variables found are not of the same length as those in newdata if it was supplied." It also says "Variables", not columns. > Or will R > just look across consecutive columns of newdata, and apply them to the > call as appropriate? No. > For example, if I have fitted a model with four variables > (x1,x2,x3,x4) in my original dataframe, and then have a second dataframe > which I want to supply to the newdata argument in predict.lm with > variable names (x5, x6, x7, x8), do I need to change the variable names > in my newdata dataframe to match those of the original dataframe? Yes. > Or > will R treat x5 as x1, x6 as x2, etc, when using predict.lm? > I would like to know so that I can design the structure of some > somewhat larger dataframes in a manner which will make using predict.lm > straight forward and quick. > Hope this makes sense. > Many thanks for any help. HTH, Marc Schwartz ______________________________________________ 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.
