Course 1.1Lesson 3 of 3
We often cannot ask everyone, so we ask a sample of the population. A fair sample is chosen by chance, so that everyone has the same chance of being asked. A sample chosen in a convenient way, for example from one place or one club, can be biased and give a wrong picture. A fair question does not push the answer.
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You want to know how tall the students of your school are. There are 800 of them. You cannot measure all 800, so you measure a few.
A sample is fair when everyone in the population has the same chance of being chosen. The best way is to choose by chance. Draw names from a hat, or let a computer choose.
A sample is biased when some people are more likely to be chosen than others. Suppose you ask only the members of the sports club whether students like sport. Club members like sport more than most, so the answer comes out too high.
You should find that the red bar stays where the club puts it, however small the club is. A smaller club makes the gap larger, because the whole school is then mostly students outside the club.
These are the heights of all 20 students of a class, in order. The program compares the whole class with two samples of 5.
The program below chooses 5 students out of 30 by chance. Press Run again and again. The students change each time, and nobody is more likely to be chosen than anybody else.
The words of a question can push the answer. "Do you agree that sleeping late is a terrible habit?" tells people what to say. A fair question is neutral: "How many hours do you sleep on a school night?"
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