Hello,
Try
df1 <- read.table(text="
PtName Visit Date Obs ObsID
S2 1 6/8/2005 KB 3
S2 1 6/8/2005 JB 5
S3 1 6/8/2005 KB 3
S3 1 6/8/2005 JB 5
D1 1 6/12/2007 CD 11
D1 1 6/12/2007 TE 2
D1 1 6/12/2007 MB 4
", header=TRUE)
head(df1)
df2 <- with(df1, sapply(split(df1, list(PtName, Date)),
function(x) if(nrow(x)) x[which(x$ObsID ==
min(x$ObsID)), ]))
df2 <- do.call(rbind, df2)
rownames(df2) <- 1:nrow(df2)
df2
Hope this helps,
Rui Barradas
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