Gong, thanks!! but your syntax did work out as I would expect. here following
are the reasons:
each row from my original data frame represents data from one subject. when
the one type I number and the
two type II numbers are drawn from the original data frame, both type I
number and type II numbers should both from the same subject. by doing so,
the new data frame should only have 3 rows with each representing one
subject. I am trying to revise your syntax to make it work... :)