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A library has 120 members, numbered 1 to 120.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A library has 120 members, numbered 1 to 120. It wants to ask 8 of them about its opening hours.
- It chooses the 8 members by chance. What is the chance that one given member is chosen? Give it as a percentage.
- Why is this a fair way to choose?
- Why is "the first 8 members who walk in on Monday morning" not a fair sample?
Need a reminder? The key ideaShow
The population is everyone we want to know about. The sample is the few we ask. A fair sample gives everyone the same chance of being chosen. A sample that leaves out a kind of person is biased.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The chance. , the same for every member.
Step 2. Why it is fair. Nobody is more likely to be chosen than anybody else.
Step 3. Monday morning. Only members who are free on a Monday morning can be chosen. Members who work or study then are left out, so the sample is biased.
Answer: . A sample chosen by chance is fair; the Monday morning sample is not.
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