`getFirstAndLastInGroup` <-
function(DT) {
VALID_CASE <- YEAR <- CONTENT_AREA <- FIRST_OBSERVATION <- LAST_OBSERVATION <- ID <- GRADE <- YEAR_WITHIN <- NULL
setkey(DT, VALID_CASE, CONTENT_AREA, YEAR, GRADE, ID, YEAR_WITHIN)
setkey(DT, VALID_CASE, CONTENT_AREA, YEAR, GRADE, ID)
DT[DT[unique(DT["VALID_CASE"], by=key(DT)),,mult="first", which=TRUE], FIRST_OBSERVATION:=1L]
DT[DT[unique(DT["VALID_CASE"], by=key(DT)),,mult="last", which=TRUE], LAST_OBSERVATION:=1L]
return(DT)
} ### END getFirstAndLastInGroup
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