The 16 students of a class scored these marks out of 50, from the lowest to the highest: 18, 21…

Step 1. The whole class. $\dfrac{510}{16} = 31.875$, about $31.9$ marks.

Step 2. The top four. $38 + 40 + 43 + 47 = 168$, and $\dfrac{168}{4} = 42$ marks. That is about 10 marks too high.

Step 3. Every fourth student. $21 + 29 + 33 + 40 = 123$, and $\dfrac{123}{4} = 30.75$ marks. That is only about 1 mark too low.

Step 4. Which is better. Every fourth student is spread over the whole class. The top four leave out everyone else, so that sample is biased.