Try this. The function takes a vector of dates of the form yyyy-mm and produces a new
character vector of dates of the same form except the
output date is the beginning of the 6 month period in which the input date lies. The
6 month intervals are measured from the minimum date.
date.grouping <- function(d) {
# for ea date in d calculate date beginning 6 month period which contains it
mat <- matrix(as.numeric(unlist(strsplit(as.character(d),"-"))),nr=2)
f <- function(x) do.call( "ISOdate", as.list(x) )
POSIXct.dates <- apply(rbind(mat,1),2,f) + ISOdate(1970,1,1)
breaks <- c(seq(from=min(POSIXct.dates), along=POSIXct.dates, by="6 mo"), Inf)
format( as.POSIXct( cut( POSIXct.dates, breaks, include.lowest=T )), "%Y-%m" )
}
patients2 <- with( patients, tapply( cost, list(ID,date.grouping(date)), sum ) )
patients2 <- as.data.frame( patients2 )
summary(patients2)
boxplot(patients2)
--- Ricardo Pietrobon <[EMAIL PROTECTED]> wrote:
>Hi,
>
>
>I am new to R, coming from a few years using Stata. I've been twisting my
>brain and checking several R and S references over the last few days to
>try to solve this data management problem: I have a data set with a unique
>patient identifier that is repeated along multiple rows, a variable with
>month of patient encounter, and a continous variable for cost of
>individual encounters. The data looks like this:
>
>ID date cost
>1 "2001-01" 200.00
>1 "2001-01" 123.94
>1 "2001-03" 100.23
>1 "2001-04" 150.34
>2 "2001-03" 296.34
>2 "2002-05" 156.36
>
>
>I would like to obtain the median costs and boxplots for the sum of
>encounters happening in the first six months after the index encounter
>(first patient encounter) for each patient, then the mean and median costs
>for the costs happening from 6 to 12 months after the index encounter, and
>so on. Notice that the first ID has two encounters during the index date,
>making it more difficult to define a single row with the index encounter.
>
>Any help would be appreciated,
>
>
>Ricardo
>
>
>Ricardo Pietrobon, MD
>Assistant Professor of Surgery
>Duke University Medical Center
>Durham, NC 27710 US
>
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>[EMAIL PROTECTED] mailing list
>https://www.stat.math.ethz.ch/mailman/listinfo/r-help
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