"Explain" answers
A standard method from Uses and abuses of statistics: the steps and worked examples.
Steps
Name the problem, then say what effect it has: "The sample includes only students in the library, so it may overestimate how long students study."
Worked examples
Example 1: Explain answers: five workers earn $15 000, $16 000, $17 000, $18 000 and $134 000 a month. A report says "the average income is $40 000". Explain why this is misleading and give a better average.
mean = 200 000 ÷ 5 = $40 000
median = $17 000The one extreme value ($134 000) pulls the mean up, so the mean is higher than what four of the five workers earn. The median, $17 000, is a fairer average here.
For the whole topic, read Uses and abuses of statistics: key points, common mistakes and how the DSE tests it.