Use positions rather than impressions
Treat the first string as the reference and the second as the copy. Start at the same edge, compare one position at a time and keep a running count. A string can look almost identical while containing several substitutions. Equally, a busy-looking pair can be an exact match.
Count differing positions, not the number of characters you would type to repair the copy. One changed character at position four is one mismatch. Use the task instructions if an exercise introduces another rule; do not silently switch between a position comparison and an editing task.
An original worked example
Reference: A7K9P2M4. Copy: A7X9P8M4. Positions one and two match. At position three, K has become X. Positions four and five match. At position six, 2 has become 8. The final two positions match. The correct mismatch count is therefore two.
Choosing one would overlook a substitution, often because the eyes jump to the most obvious letter change. Choosing three might mean counting a position twice or treating the changed reference and changed copy as separate errors. The explanation should identify the exact positions so you can check your method.
Improve accuracy before increasing speed
Compare in short, fixed groups without changing alignment. If the strings are long, mentally segment both at the same boundaries. Keep the group size small enough to check precisely. Looking back over the whole line repeatedly can consume time without revealing which position you missed.
During review, name the error you made. A missed mismatch, a false mismatch and a lost place are different problems. Practise a fresh item using the correction, then test pace separately. Deliberate review of an old question is useful, but it should not be confused with a fresh assessment of your checking skill.
What PSP feedback should show
Feedback reconstructs the comparison, gives the correct count and explains the plausible incorrect counts. Use the aligned character positions rather than colour alone so the explanation remains understandable if a highlight is difficult to distinguish.
PSP timing and scoring are practice settings. The official invitation decides the live instructions and whether this format appears. If a practice extension uses a longer string or a higher working-memory demand, treat it as additional training rather than an official specification.
Questions about this stage
Can the answer be zero?
Yes, when every corresponding character matches.
Should I memorise the strings?
Use the presentation rules of the exercise. The core method is a consistent position-by-position comparison, not memorising familiar answers.
Sources and current instructions
These guides explain the preparation process. Check your current competition booklet and invitation before relying on a requirement or date.
Official decisions, eligibility, medical clearance and assessment results belong to the recruiting organisation. A PSP practice score is not an official pass mark or selection prediction.