Anticipating problems with the sample and their impact

Overview

Sampling bias, under-covered group and sampling error.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define sampling bias.

   Model answer

   Sampling bias is a systematic error that makes the sample differ from the population it is meant to represent.

2. 2

   How does an incomplete sampling frame bias a study?

   Model answer

   Anyone missing from the list can never be selected, so whole groups are excluded and the sample misrepresents the population.

3. 3

   Why is the electoral register an imperfect sampling frame?

   Model answer

   People who have not registered, who move often or who are not eligible to vote are absent from it, and they are not a random slice of the population.

4. 4

   What impact does refusal have on a sample's representativeness?

   Model answer

   Refusals are rarely random, so the achieved sample over-represents the people who are most willing to be studied.

5. 5

   What is an under-covered group, and what does it do to the findings?

   Model answer

   It is a group present in the population but scarce in the sample, so any conclusion about it rests on too few cases to be reliable.

6. 6

   Why can a sample be representative overall yet useless for comparison?

   Model answer

   If a subgroup amounts to only a handful of cases the total may match the population while that subgroup is far too small to analyse.

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