I checked different webs and books, amazingly the def. are so different !!
for example, GRE official said quartile is one of the four groups.
so you got first, second, third, and fourth quartile in a series.
but other web said it is the specific single value that cut series to four groups.
so you only got Q1,Q2,Q3.
whether the quartile is a group of numbers or a number !!????
thx