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The perfidy of percentiles
1Department of Psychology, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6. bowman@sfu.ca
Summary
Psychology professionals often misunderstand percentile ranks (PRs), mistakenly viewing them as equal intervals. This leads to significant misinterpretations in forensic and educational psychology, impacting legal and clinical decisions.
Area of Science:
- Psychology
- Neuropsychology
- Forensic Psychology
- Educational Psychology
Background:
- Standard texts recommend percentile ranks (PRs) for their perceived ease of understanding by non-psychologists.
- This recommendation is based on the assumption that PRs are intuitively grasped.
- However, the actual interpretation of PRs may be flawed.
Purpose of the Study:
- To test the assumption that percentile ranks (PRs) are easily understood by non-psychologists.
- To investigate systematic errors in interpreting PRs, particularly when compared to IQ scores.
- To examine the implications of these interpretation errors in forensic neuropsychology.
Main Methods:
- A 12-item task was designed to assess interpretation of PR scores.
- Third-year psychology undergraduates compared PR scores with IQ metrics.
- Four hypotheses regarding systematic interpretation errors were tested.
Main Results:
- All hypotheses predicting significant, asymmetrical errors in PR interpretation were supported.
- Even subjects with psychometric education demonstrated gross misinterpretations of PR meaning.
- Common graphical displays may exacerbate these interpretive errors.
Conclusions:
- The assumption that percentile ranks (PRs) are easily understood is incorrect.
- Misinterpretation of PRs in forensic neuropsychology can lead to distorted legal awards (e.g., compensation, personal injury).
- Misinterpretation of PRs for risk assessment can negatively impact parole decisions.