[R] RES: another aov results interpretation question

[Gabor Grothendieck]

On 6/20/05, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> On 6/20/05, Paulo Brando <pmbrando at ipam.org.br> wrote:
> >
> > Dear All,
> >
> > I created a script to calculate averages - two groups: "parcel" and
> > "date" - and, based on these averages, make a graph. The problem is that
> > 'R' does not recognize the first column even if I try to insert one.
> >
> > A brief example
> >
> > Raw data:
> >
> > Data <- sample(1:100, 30, replace = FALSE, prob = NULL)
> > Date <- rep(c("02/30/2004","03/15/2004", "04/16/2004", "06/14/2004",
> > "07/08/2004"), 6)
> > Parcel <- rep(c("DRY", "CONTROL"),15)
> >
> > Df <- data.frame(Parcel, Date, Data)
> >
> > Res <- tapply(Df$Data, list(Date = Df$Date, Parcel = Df$Parcel), mean)
> >
> >
> > > Res
> >            Parcel
> > Date          CONTROL      DRY
> >  02/30/2004 53.00000 52.33333
> >  03/15/2004 54.00000 67.66667
> >  04/16/2004 54.66667 30.00000
> >  06/14/2004 27.66667 20.00000
> >  07/08/2004 59.00000 38.00000
> >
> > > colnames(Res)
> > [1] "CONTROL" "DRY"
> >
> > > matplot(Res[,1], Res[,-1], type = "l")
> >
> > ### It does not recognize the colunm "Date". Why?
> 
> Date is a not a column in the matrix -- it is the name
> of the row dimension (and Parcel is the name of the
> column dimension).   Res is a 5 by 2 matrix whose
> colnames are CONTROL and DRY.  Try this:
> 
>   dimnames(Res)
>   dim(Res)  # 5 by 2
> 
> Also note that the first date (Feb 30th) is illegal and matplot
> does not work with dates in any case.  Fix up the erroneous
> date and convert the row names to an object, xx, of chron or Date
> class, using plot for the axes and matlines for the lines:
> 
>   xx <- format(rownames(Res), "%m/%d/%Y")  # Date class

That should read:

     xx <- as.Date(rownames(Res), "%m/%d/%Y") # Date class

>   plot(range(xx), range(Res), type = "n")  # axes
>   matlines(xx, Res)
> 
> More on dates is in R News 4/1.
>