Run a fair survey. Pick a yes-or-no question. Now take a list of everyone in a group, such as a class list or a street, and choose every third or every tenth name. Ask only those people, using the same words each time. Then try asking only your friends. Do the answers come out the same?

Before chapter 3. The flyer says nine out of ten pupils love the menu. Who were the ten?

The answer. They were the ten members of the food committee, who chose the menu. Their survey was true about them, but not about the school. Picking every tenth name from the 300-pupil register gave 30 pupils, and 18 of them liked the menu: 60 per cent, or six in ten.

The grown-up layer. "Who was asked?" is the first question to ask about any percentage, in an advert or the news. DataForge's Cull and NewsForge's Source both work on it.

For the ride home. Ask: if you only asked people at the swimming pool whether they like swimming, what might you find?