Every standard score, on one curve.
The exam moves between z-scores, T-scores, deviation IQs, stanines and percentile ranks. Enter any one and see the others and where it sits on the normal curve. Then drill the conversions until they are automatic.
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T-score{{ t }}
Deviation IQ{{ iq }}
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Stanine{{ stanine }}
Percentile rank{{ pct }}
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The shaded bands hold about 68, 95 and 99.7 percent of scores: within one, two and three standard deviations of the mean.
The conversions to know cold
Memorize one row, z of +1: T 60, IQ 115, about the 84th percentile. Every other row is a step up or down from it.
| z-score | T-score | Deviation IQ | Percentile rank, about |
|---|---|---|---|
| +2 | 70 | 130 | 98th |
| +1 | 60 | 115 | 84th |
| 0 | 50 | 100 | 50th |
| −1 | 40 | 85 | 16th |
| −2 | 30 | 70 | 2nd |
- A percentile rank is the percentage of the norm group scoring at or below a score. It is not equal-interval: ten percentile points near the middle are a much smaller difference than ten near the tails.
- Percentile and percent correct are different things.
- Stanines run 1 to 9 with a mean of 5 and an SD of about 2. Each covers half a standard deviation, except the two ends.
- The Wechsler scales use mean 100, SD 15. The Stanford-Binet historically used 16 and now uses 15.
From the lessons Norms, scales of measurement, standard scores and percentiles and Descriptive statistics.
Drill: ten quick conversions
Answer before you check. The answer order changes every time.
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