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.
PROMIS bands (under 55 normal, 55–59 mild, 60–69 moderate, 70+ severe) checked against HealthMeasures guidance and the APA's DSM-5-TR Level 2 depression measure; one trial-based study also proposes 65 for moderately severe (Kroenke et al., 2020). For Heaton-style norms, the key cut-off (T below 40 = impaired, i.e. more than 1 SD below demographically corrected expectations) is confirmed in studies citing Heaton et al. 1991 and 2004; the finer sub-bands below 40 are as commonly reproduced; individual manuals and clinicians may use different cut-offs. Not a diagnostic tool.
Not every T-score sits on a bell curve
This converter assumes a normalised T: raw scores forced onto the normal curve, so T 70 is always the 97.7th percentile. IQ composites work this way.
Many symptom scales do not. Most people score near zero symptoms, so raw scores pile up at the bottom with a long tail. A linear T keeps that shape, and a 70 may be rarer or commoner than 2.3%. The MMPI-2 uses "uniform" T-scores, a compromise that lines up percentiles across scales without forcing them 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.