[R] Selecting groups with R

jlwoodard john.woodard at wayne.edu
Sat Aug 22 00:08:09 CEST 2009

I have a data set similar to the following:

Color  Score
RED      10
RED      13
RED      12
WHITE   22
WHITE   27
WHITE   25
BLUE     18
BLUE     17
BLUE     16

and I am trying to to select just the values of Color that are equal to RED
or WHITE, excluding the BLUE.

I've tried the following:
myComp1<-subset(dataset, Color =="RED" | Color == "WHITE")
myComp1<-subset(dataset, Color != "BLUE")
myComp1<-dataset[which(dataset$Color != "BLUE"),]

Each of the above lines successfully excludes the BLUE subjects, but the
"BLUE" category is still present in my data set; that is, if I try
table(Color)  I get 

82     151      0

If I try to do a t-test (since I've presumably gone from three groups to two
groups), I get:
Error in if (stderr < 10 * .Machine$double.eps * max(abs(mx), abs(my)))
stop("data are essentially constant") : 
  missing value where TRUE/FALSE needed
In addition: Warning message:
In mean.default(y) : argument is not numeric or logical: returning NA

and describe.by(score,Color) gives me descriptives for RED and WHITE, and
BLUE also shows up as NULL.

How can I eliminate the BLUE category completely so I can do a t-test using
Color (with just the RED and WHITE subjects)?

Many thanks in advance!!


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