Plot cumulative frequency at the UPPER class boundary
A cumulative frequency point is plotted at the end of each class (the upper boundary), against the running total — never at the midpoint.
Read quartiles off the curve at n/2, n/4 and 3n/4
On a cumulative frequency graph: median at , lower quartile at , upper quartile at ; go up to the curve, then across.
IQR is a subtraction
Interquartile range . A box plot shows min, LQ (lower quartile), median, UQ (upper quartile), max — range .
Drawn from real examiner reports.
Plotting at midpoints (or drawing a bar chart)
Plotting cumulative frequency at the mid-interval value instead of the upper boundary, or drawing bars instead of a curve.
Adding the quartiles instead of subtracting
Giving UQ (upper quartile) + LQ (lower quartile) for the interquartile range rather than UQ − LQ.
Confusing the median with a quartile
Reading the lower quartile value when the median is asked for, or finding ¼ and ¾ of n rather than reading the curve.
"More than" questions: forgetting to subtract from the total
For "how many weigh more than X", read the cumulative frequency at X and subtract from the total (and give a whole number).
Show your read-off lines on the graph
Draw the construction lines up to the curve and across to the axis — they earn method marks even if the final read-off slips.
Cumulative frequency is a running total. Plot each point at the upper boundary of the class against the running total, then join with a smooth curve.
To read statistics off the curve (with values), go up to the curve then across:
A box plot shows which five values?
For the ordered data (which has values), find the median, the lower and upper quartiles, and the interquartile range.
