Every 10 T-points is one SD, or 15 IQ points
The T-scale was proposed by William McCall in 1922 for school testing and named in honour of Edward Thorndike and Lewis Terman. He wanted a scale with no negative numbers that still spread most children across a usable range: 20 to 80 covers ±3 SD, more than 99.7% of people.
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T 65 on three kinds of test
Typical of IQ composites expressed as T, and of many memory and achievement subtests. T 50 is the population median.
PROMIS anxiety, depression and fatigue scores are T-scores centred on the US general population; published guidance treats 55, 60 and 70 as mild, moderate and severe thresholds.
Demographically corrected T-scores (age, education, sex) as in Heaton and colleagues' norms. A T of 40 for a graduate can be a different raw score than for a school-leaver.
Those symptom bands come from particular manuals, not from T-scores themselves. HealthMeasures and the DSM-5-TR Level 2 measure use 55, 60 and 70 as common PROMIS guideposts; a study by Kroenke and colleagues also uses 65 for moderately severe symptoms. Heaton-style neuropsychology norms commonly flag T below 40, but manuals and clinicians can set different cut-offs. This tool converts scales; it does not diagnose.
Not every T-score sits on a bell curve
This tool uses a normalised T-score: raw results are mapped onto a normal curve, so T 70 always lands at the 97.7th percentile. IQ composites use this kind of conversion.
Symptom scales are often different. Most people report few symptoms, leaving a long right tail rather than a bell curve. A linear T preserves that shape, so T 70 can be rarer, or less rare, than 2.3%. The MMPI-2 uses uniform T-scores to align percentiles across scales without making every distribution normal.
- McCall, W. A. (1922). How to Measure in Education. New York: Macmillan. Introduces the T-scale.
- Butcher, J. N., Dahlstrom, W. G., Graham, J. R., Tellegen, A., & Kaemmer, B. (1989). MMPI-2 Manual for Administration and Scoring. University of Minnesota Press. Uniform T-scores.
- Cella, D., et al. (2010). The PROMIS first wave of adult self-reported health outcome item banks. Journal of Clinical Epidemiology, 63(11), 1179–1194.
- Heaton, R. K., Miller, S. W., Taylor, M. J., & Grant, I. (2004). Revised Comprehensive Norms for an Expanded Halstead-Reitan Battery. PAR.
- Tellegen, A., & Ben-Porath, Y. S. (1992). The new uniform T scores for the MMPI-2: Rationale, derivation, and appraisal. Psychological Assessment, 4(2), 145–155.