Meaning
Why a 15-point standard deviation describes spread and connects scores to percentiles.
Interpretation depends on population, instrument, edition, year, sampling, language, and method.
Read charts carefully
Check representativeness, uncertainty, exclusions, and whether values were measured or estimated. Decimal-heavy rankings are not automatically precise.
Group averages have substantial overlap and cannot determine an individual.
Responsible conclusion
Prefer transparent primary datasets and intervals. Avoid causal claims from correlation alone.
For an individual, use appropriate age norms and technical documentation.
COMMON QUESTIONS
Frequently asked questions
REFERENCES