Sampling and Representativeness

UCAT Decision Making questions sometimes ask you to evaluate whether a conclusion drawn from data is justified. A key factor is whether the sample (the group studied) is representative of the population (the group you want to draw conclusions about). This lesson teaches you to identify sampling bias, evaluate the strength of evidence, and recognise when data supports a conclusion — and when it does not.

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Populations and Samples

Term	Definition	Example
Population	The entire group you want to draw conclusions about	All NHS patients in England
Sample	The subset of the population that was actually studied	500 patients at three London hospitals

The purpose of a sample is to learn something about the population without having to study every member. For this to work, the sample must be representative — it must reflect the characteristics of the population.

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Types of Sampling Bias

Selection Bias

The sample is chosen in a way that systematically excludes certain groups.