Biostatistics and epidemiology
Data types, summary measures and the normal distribution
Types of data, mean, median, standard deviation and interquartile range, skew, and how the normal distribution sets reference ranges and DXA T-scores.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Choose the summary by the type of data and its shape. Symmetric numerical data: mean and SD. Skewed data or ordinal scores: median and interquartile range (IQR).
- In a normal distribution, mean = median = mode, and 95% of values lie within 1.96 SD of the mean.
- Most laboratory reference ranges are the central 95% of a healthy population, so 1 in 20 healthy people is "abnormal" on any one test.
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Sources
- Osteoporosis or low bone mass at the femur neck or lumbar spine in older adults: United States, 2005-2010CDC National Center for Health Statistics (Looker and Frenk), 2015
- Practical Statistics for Medical ResearchChapman and Hall (Altman, 1991), 1991
- Essential Medical Statistics (2nd ed.)Blackwell (Kirkwood and Sterne, 2003), 2003
- Statistics notes: The normal distributionBMJ (Altman and Bland, 1995;310:298), 1995
- Statistics notes: Transforming dataBMJ (Bland and Altman, 1996;312:770), 1996
- ISCD Official Positions: AdultInternational Society for Clinical Densitometry, 2023
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Personal emails, About story and Australian-style copy