How to play
PASAT Explained: How the Serial Addition Test Works
By HumanCalc · Published · 3 min read
The PASAT (Paced Auditory Serial Addition Test) plays a series of single digits at a fixed pace. After each new digit you say the sum of that digit and the one just before it. Dorothy Gronwall published it in 1977 to track how people recovered after concussion, and clinicians have used it since to assess processing speed and sustained attention.
The rule is simple to state and hard to keep, because the obvious habit, keeping a running total, gives the wrong answer from the third digit on.
A worked example
Digits: 3, 5, 2, 7, 4
- After 5: 3 + 5 = 8.
- After 2: 5 + 2 = 7. (Not 8 + 2.)
- After 7: 2 + 7 = 9.
- After 4: 7 + 4 = 11.
Answers: 8, 7, 9, 11. Each answer uses only the last two digits you heard.
The mistake almost everyone makes
The common error is adding the new digit to your last answer. With the digits above, that gives 8, 10, 17 and 21. It feels natural because every other adding task works that way.
The trick is to keep the last digit in mind and let your answer go as soon as you have given it. Say the answer, then repeat the newest digit to yourself while you wait for the next one.
Why the pace makes it hard
In the original test the digits come on a recording at a fixed interval, and later trials use shorter intervals. If you are still working on one sum when the next digit arrives, you miss it, and recovering takes a turn or two. Tombaugh's 2006 review covers the different versions of the test, how it is scored and the published norms.
Many people find the test stressful, including people with nothing wrong with them. Struggling with it at the faster paces is normal.
HumanCalc's practice version
Among 58 HumanCalc players up to 26 October 2026, the median best level was 4. Fourteen of them had a best of level 1. This is a practice game built on the same rule. It is not the clinical PASAT and gives no clinical result.
- Numbers appear on screen, one at a time. After each one, pick the sum of it and the previous number from four choices.
- A round has 16 numbers, so 15 answers, and you have three lives.
- There are 12 levels. The time to answer drops from 2.3 seconds at level 1 to 0.4 seconds at level 12, and the numbers get larger.
- You can answer with the number keys 1 to 4 or by tapping.
Research sources
Try it now
Play Serial Addition in your browser
Add each number to the one before it — not the total. About 4 min. Free, no signup.
Play Serial Addition