Posted by Anonymous on .
I am having trouble with finding the 1st and 3rd quartiles of a set of data. For example, let's say the data is:
4, 9, 11, 16, and 24< these are cumulative frequencies. The median (or 2nd percentile)is 11, right? My teacher says it's 12. Then for 3rd quartile I got 20, but my teacher says 18. Can someone explain to me what I am doing wrong?
Thanks.

Algebra 
PsyDAG,
The median is always the 50th percentile or 2nd quartile.
You seem to give frequencies (Yaxis/ordinate values), but do not give the quality/quantity in which these frequencies occur (Xaxis/abscissa).
These are frequencies of what? Height? Weight? Test scores?
The quartiles would be given in terms of the abscissa values.
I hope this helps. If not, repost with more complete data. Thanks for asking.