You can do something like this sp<-split(dat, list(dat$Vial,dat$Site))
seq.model<-lapply(sp, function(x) lm(x$InRFU~x$Time)) Then, extract whatever you want from seq.model Weidong Gu On Mon, Aug 22, 2011 at 9:15 PM, Nathan Miller <natemille...@gmail.com> wrote: > Hi all, > > I have a data set that looks a bit like this. > > feed1 > RFU Site Vial Time lnRFU > 1 44448 1 1 10 10.702075 > 2 47521 1 1 20 10.768927 > 3 42905 1 1 30 10.66674 > 4 46867 1 1 40 10.755069 > 5 42995 1 1 50 10.668839 > 6 43074 1 1 60 10.670675 > 7 41195 1 1 70 10.626072 > 8 47090 1 2 10 10.759816 > 9 48100 1 2 20 10.781037 > 10 43215 1 2 30 10.673943 > 11 39656 1 2 40 10.587998 > 12 38799 1 2 50 10.566150 > 13 38424 1 2 60 10.556438 > 14 35240 1 2 70 10.469937 > 15 46427 1 3 10 10.745636 > 16 46418 1 3 20 10.745443 > 17 42095 1 3 30 10.647684 > ...... > There are 5 columns of data, three levels of "Site", 10 "Vials" per site, > and measurements were taken at 10 min intervals from 10-70.. I am primarily > interested in the relationship between "Time" and "lnRFU" to calculate the > rate at which lnRFU declines over time. I have a nice plot using a ggplot2 > code that looks like this > > p<-ggplot(data=feed1,aes(x=Time,y=lnRFU)) > p+geom_point(size=4)+facet_grid(Site~Vial)+geom_smooth(method="lm") > > The graph is useful to visualize the changes over time and grouped by both > Site and Vial, but I also need the slopes of the linear regressions for each > Vial, within a Site. This is where I run into a problem. I want to run a > linear regression of lnRFU as a function of Time grouped by both Site and > Vial. Its easy to visualize this comparison in ggplot using facet_grid(), > but I'm not sure how to do a similar comparison/analysis within lm() > > I imagine something like > > fit<-lm(lnRFU~Time | Vial * Site, data=feed1) > > in which I group by both Vial and Site, but obviously this code doesn't > work. Does anyone have an idea for how to do a linear regression with two > grouping variables? Do I have to go back and combine Vial and Site into a > single grouping variable or can I leave the dataframe the way it is? I'm > trying to imagine a means of accomplishing the same type of thing that > facet_grid does when it allows you to plot the data as a function of two > "grouping" variables. > > Thanks for you time. I greatly appreciate it. > > Nate Miller > > [[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. > ______________________________________________ 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.
