SPM Mathematics Glossary
Measures of Dispersion for Grouped Data, terms
Frequently asked questions
What's the difference between class midpoint and class interval, and why does mixing them up cause errors?
Class interval is the range of values a group of data covers, such as 10–19, while class midpoint is the single representative value at its centre, used when calculating the mean. Using the interval instead of the midpoint in a mean calculation is a common mistake that throws off the whole answer.
How are quartiles found from an ogive, and what does "interpolation" mean here?
Quartiles are read off the ogive at one quarter, half, and three quarters of the total cumulative frequency on the vertical axis, tracing across to the curve and down to the horizontal axis. Interpolation means estimating a value that falls between two known data points on the graph, rather than reading an exact plotted point.
Which term should I get comfortable with first when starting this topic, ogive, histogram, or modal class?
Start with histogram and modal class, since they build directly on frequency tables you already know from earlier grouped-data work. Once you can identify the modal class from bar heights, move to the ogive, which needs cumulative frequency first, get that running total right before attempting quartiles or interpolation.