Test types

The WAIS

The Wechsler Adult Intelligence Scale is the reference instrument for adult cognitive assessment in English-speaking countries. It takes an hour or two and a licensed psychologist.

What it is

David Wechsler published the first version in 1939 at Bellevue Hospital in New York. The current edition is the fifth, and the test has been the standard adult instrument for most of the intervening period.

Two decisions made it the standard. Wechsler abandoned the mental-age ratio in favour of the deviation IQ — your position within your own age group — which is what every modern test now uses. And he reported a profile of separate abilities rather than a single number, on the grounds that the pattern is clinically more useful than the total.

What it measures

The modern WAIS is organised into index scores, each built from several subtests, with a full-scale IQ aggregating them.

Each index is reported on the familiar scale with a mean of 100 and a standard deviation of 15. Subtests use a scaled score with a mean of 10 and a standard deviation of 3.

What a session is actually like

One to two hours, one-to-one, in a quiet room. The psychologist reads standardised instructions, presents materials, times what needs timing, and records both your answers and how you produced them.

The observation is not incidental. How someone approaches a block design task — systematically, by trial and error, giving up early, persisting past the time limit — is clinical information that no score captures, and it is a large part of why individual administration is worth the cost.

Subtests stop when you reach a ceiling of consecutive failures, so nobody works through the full range. Most people find parts of it easy and parts impossible, which is how it is designed.

What the report says

A properly written report contains more than a number, and knowing what to expect is useful if you are commissioning one.

• The full-scale IQ with its 95 percent confidence interval and percentile.
• Each index score with its own interval and percentile.
• A discussion of whether index differences are large enough to be meaningful — they often are not.
• Observations about how you approached the tasks.
• Interpretation in the context of why the assessment was requested.

The confidence interval is the part that matters most and is most often dropped when a result gets quoted afterwards. A full-scale score carries an interval of roughly plus or minus five points; index scores are shorter and noisier, often eight to ten points wide.

When you would need one

Assessment for a learning disability or intellectual disability. Neuropsychological evaluation after injury, stroke or illness. Documentation for an examination or workplace accommodation. Forensic contexts. Occasionally, pre-surgical or diagnostic work where cognitive baseline matters.

It is not something people generally take out of curiosity, because it costs several hundred units of whatever currency you use and requires a referral in many systems. If curiosity is the motive, a free test is the proportionate option — as long as nobody pretends the two are equivalent.