"Sampling" basically means selecting people/objects from a
"population" in order to test the population for something. For example, we
might want to find out how people are going to vote at the next election.
Obviously we can't ask everyone in the country, so we ask a sample.
When considering a particular population it is usually
advisable to choose a sample in such a way that everyone is represented. This is
not easy and requires careful thought about sample size and composition. Often
questionnaires are devised to identify the required information. These need to
be idiot proof, so questions need to cover all alternatives and give little
scope for variation.
Example question
A bus company attempted to estimate the number of people who
travel on local buses in a certain town. They telephoned 100 people in the town
one evening and asked 'Have you travelled by bus in the last week?' Nineteen
people said 'Yes'. The bus company concluded that 19% of the town's population
travel on local buses. Give 3 criticisms of this method of
estimation.
In answering this question, there are no 3 correct answers.
As long as what you say is plausible and sensible, you should get the marks. For
example, you might say:

100 people in a large town is not a large enough proportion
of the population to give a good sample.

People who travel on local buses once a fortnight may have
said no to the question. They nevertheless travel on local buses.

On the evening that the sample was carried out, anybody
travelling by bus would be out.
Copyright © Matthew Pinkney 2003
