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Restriction of range is a statistical issue that occurs when one or both variables in a correlational study have a limited range of values in the sample relative to the actual population. This artificial limitation can cause a strong overall correlation to appear misleadingly weak or even non-existent. To prevent this, researchers should aim to collect data from subjects spanning a wide range of values for their primary variables.

Restriction of Range

The statistical relationship between age and the enjoyment of hip hop music provides a clear example of how restriction of range can mislead researchers. Across the general population, there may be a strong negative correlation (e.g., $$r = -0.77$$) between these variables. However, if data is collected exclusively from a restricted age group—such as $$18$$- to $$24$$-year-olds—the relationship appears artificially weak, dropping to a Pearson's $$r$$ of $$0$$. This demonstrates the importance of sampling a wide range of values to accurately capture overall trends and avoid obscured results.

Example of Restriction of Range: Age and Hip Hop Music

Which of the following best describes the statistical issue known as 'restriction of range'?

A psychologist investigates the relationship between anxiety levels and test performance but recruits only participants whose anxiety scores fall within a narrow, moderate range. If a strong negative correlation between anxiety and test performance exists in the broader population, the psychologist's restricted sample is likely to produce a weaker correlation coefficient than what would be observed using a more representative sample.

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