I am trying to write a function which will allow me to analyze many variables
all of which have the same form, Base.stem and Disch.stem, where stem varies
from variable to variable, e.g. Base.rolling and Disch.rolling, Base.standing,
Disch.standing.
I want to pass a dataframe and the stem of the name of the pre and post
intervention variables. In my function I create the full pre and post
intervention Base.stem and Disch.stem variables names (by using the paste
function, varpre=paste("Base.",stem,sep="") and
varpost=paste("Disch.",stem,sep="")). I also create and add to the dataframe
the change variable (i.e. post-pre) formed as ch<-paste("Change.",stem,sep="")
and compute the change in the variable:
data[,ch]=data[,varpo]-data[,varpre].
If, for example, I run the function doit(rolling, data), the function creates
the full pre-intervention variable name Base.rolling, the post-intervention
variable name Disch.rolling, and computes Change.rolling =
Disch.rolling-Base.rolling. I want to use Change.rolling in a function, but I
don't want to have to expressly specify the name of the outcome variable, i.e.
fitwilcox<-wilcox.test(Change.rolling~RANDOMIZED,data=data,na.action=na.omit)
I want to specify the constructed variable, i.e.
fitwilcox<-wilcox.test(ch~RANDOMIZED,data=data,na.action=na.omit)
This does not work because ch is not in the dataframe, but Change.rolling is.
How can I modify the call to the Wilcox.test function so I can specify the
change variable constructor. I tried the following but it does not work:
fitwilcox<-wilcox.test(eval(ch,
parent.frame())~RANDOMIZED,data=data,na.action=na.omit)
DATA:
> dput(data2)structure(list(Subj = c(115, 116, 117, 118, 119, 120, 121, 122,
> 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137,
> 138, 139, 140, 141, 142, 143, 144, 145, 146, 147), RANDOMIZED = c(1, 1,
> 2, 1, 2, 1, 2, 1, 1, 2, 1, 2, 1, 2, 1, 1, 2, 2, 1, 2, 1, 1, 2, 2, 1, 2,
> 1, 2, 2, 2, 2, 2, 2), Base.rolling = c(0, 6, 6, 3, 3, 1, 4, 6, 4, 6, 4,
> 1, 3, NA, 1, 2, 4, 1, 4, 4, 4, 1, 5, 3, 2, 3, 1, 4, 0, 4, 1, NA, 3),
> Disch.rolling = c(6, NA, NA, 6, 6, 3, 6, 6, 6, NA, NA, 6, 1, NA, 6, 6, 6,
> NA, 6, 6, 6, NA, 4, NA, 6, 6, NA, 6, NA, 6, 6, NA, NA)), .Names =
> c("Subj", "RANDOMIZED", "Base.rolling", "Disch.rolling"), class =
> "data.frame", row.names = c(NA, -33L))
CODE:
doit <- function(var,data){
var <- deparse(substitute(var))
print(var)
# Create name for basline variable.
varpre<-paste("Base.",var,sep="")
# Create name for post variable.
varpo<-paste("Disch.",var,sep="")
# Create a name for the change variable.
ch<-paste("Change.",var,sep="")
print(ch)
# Compute change.
data[,ch]=data[,varpo]-data[,varpre]
cat("\nData used in the analyses\n")
print(data[,c("Subj","RANDOMIZED",ch)])
# This works when I expressly specify the change variable.
cat("This works, expressly specify change variable\n")
fitwilcox1<-wilcox.test(Change.rolling~RANDOMIZED,data=data,na.action=na.omit)
print(fitwilcox1)
# This does not work.
cat("This does NOT work, try to use created change variable.\n")
fitwilcox2<-wilcox.test(eval(ch,
parent.frame())~RANDOMIZED,data=data,na.action=na.omit)
print(fitwilcox2)
# Compute wilcoxon statistic.
}
doit(rolling,data2)
Thank you,
John
John David Sorkin M.D., Ph.D.
Professor of Medicine
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology and Geriatric
Medicine
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
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