[R] Selecting groups with R

[Don McKenzie]

Right, but he just wanted to eliminate "BLUE" as far as I could see.   
Your solution does more, of course.


On 21-Aug-09, at 3:33 PM, David Winsemius wrote:

>
> On Aug 21, 2009, at 6:16 PM, Don McKenzie wrote:
>
>> dataset[dataset$Color != "BLUE",]
>
> Will return a data.frame with Color still a factor with three levels.
>
>>
>> On 21-Aug-09, at 3:08 PM, jlwoodard wrote:
>>
>>>
>>> I have a data set similar to the following:
>>>
>>> Color  Score
>>> RED      10
>>> RED      13
>>> RED      12
>>> WHITE   22
>>> WHITE   27
>>> WHITE   25
>>> BLUE     18
>>> BLUE     17
>>> BLUE     16
>>>
>>> and I am trying to to select just the values of Color that are  
>>> equal to RED
>>> or WHITE, excluding the BLUE.
>>>
>>> I've tried the following:
>>> myComp1<-subset(dataset, Color =="RED" | Color == "WHITE")
>>> myComp1<-subset(dataset, Color != "BLUE")
>>> myComp1<-dataset[which(dataset$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
>>>
>>> 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)?
>
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
>

Don McKenzie, Research Ecologist
Pacific WIldland Fire Sciences Lab
US Forest Service

Affiliate Professor
School of Forest Resources, College of the Environment
CSES Climate Impacts Group
University of Washington

desk: 206-732-7824
cell: 206-321-5966
dmck at u.washington.edu
donaldmckenzie at fs.fed.us