[R] Linear Regression with 2 grouping variables

[Weidong Gu]

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 <natemiller77 at 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]]
>
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