Collecting, displaying and comparing data

Discrete or continuous?

Discrete data can only take separate values (number of brothers, shoe size). Continuous data can take any value in a range (height, time, mass). Discrete data suits a bar chart with gaps; continuous data suits a grouped frequency table and a histogram-style chart with no gaps.

Grouping data

Groups must not overlap and must leave no gaps: $0\le h<10$, $10\le h<20$, and so on. Writing $0-10$ then $10-20$ is ambiguous — where does $10$ go? Cambridge marks that as an error.

Comparing two sets

Always compare two things: an average (which set is higher on the whole) and the range (which set is more spread out). A full answer says both, in context: “Class A had a higher mean, so they scored better on average, but Class B had a smaller range, so their scores were more consistent.”