On Aug 21, 2009, at 6:08 PM, jlwoodard wrote:
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$Color != "BLUE"),]
Each of the above lines successfully excludes the BLUE subjects, but
the
"BLUE" category is still present in my data set; that is, if I try
table(Color) I get
RED WHITE BLUE
82 151 0
You are being bitten by the behavior of factors.
If I try to do a t-test (since I've presumably gone from three
groups to two
groups), I get:
How.... did you do the "t-test"?
Error in if (stderr < 10 * .Machine$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)?
dataset$Color <- as.character(dataset$Color)
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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