ACT Error Detection guide: letter checking at speed
Updated 9 September 2026 · 5 min read
Error Detection is the ACT section formerly known as Letter Checking. It is the purest speed test of the five, and the one where the gap between practised and unpractised applicants shows up most clearly.
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The format
You are shown pairs of letters, arranged one above the other in two rows. You count how many pairs are the same letter. That is all.
The one rule people forget under pressure: case does not matter. An upper-case A above a lower-case a is a match. The test is not asking whether the characters look identical, it is asking whether they are the same letter.
Why it is harder than it sounds
Nobody gets these wrong through lack of ability. They get them wrong for two reasons.
Look-alike pairs. The questions are seeded with letters that are easy to confuse at a glance: b and d, p and q, m and n, u and v, i and l. Anyone skimming will read what they expect rather than what is there.
Losing count. By the time you have checked the fourth pair, it is surprisingly easy to have lost track of whether you were on two or three. The counting is a genuine part of the task, not an afterthought.
A method that holds up
- Work left to right, once. Fix a direction and never backtrack. Re-checking a pair you have already done is the most expensive habit in this section, and it almost never changes your answer.
- Say the letter, not the shape. Naming each letter to yourself, even silently, forces you to identify it rather than pattern-match it. This is what kills the b-and-d error.
- Keep the count physically. Many people find it easier to track the running total on their fingers than in their head, which frees up attention for the letters themselves.
- Commit and move on. If you are unsure, answer anyway. A question you dwell on costs you the next two.
Accuracy versus speed
It is worth being explicit about this, because the instinct to be careful is a trap here. Your ACT score reflects speed as well as accuracy. Slowing down until you are certain of every answer is not the safe option — it is a different way of scoring badly.
What you are looking for is the fastest pace at which your accuracy still holds up, and the only way to find that is to practise at speed and watch where it breaks. Try our error detection practice, which runs to a clock by default.
For how this section fits with the other four, read the Army Cognitive Test explained.
Frequently asked questions
What is Error Detection on the Army Cognitive Test?
It is the letter-checking section. You are shown pairs of letters arranged one above the other and count how many pairs are the same letter.
Does upper and lower case matter in Error Detection?
No. An upper-case letter above the lower-case version of the same letter counts as a match. The section asks whether the letters are the same, not whether the characters look identical.
How do I get faster at letter checking?
Work in one direction without backtracking, name each letter to yourself rather than recognising its shape, keep the running count somewhere reliable such as on your fingers, and answer rather than dwelling when you are unsure.
Is forcesready.co.uk affiliated with the British Army?
No. forcesready.co.uk provides independent practice materials and is not affiliated with, endorsed by, or associated with the British Army or the Ministry of Defence.
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