"Silly mistake" is the wrong name. Nobody drops a sign because they are silly; they drop it because a specific habit is missing at a specific step. Find the step, add the habit, and the mistake stops. Here are the six that account for almost all of them.
The six causes
- Sign errors — usually when moving a term across the equals sign or expanding a bracket with a minus in front.
- Copying errors — the question says 3.5, your working says 3.6; the previous line says 2x, the next line says 2.
- Unit and conversion errors — cm mixed with m, minutes with seconds, a percentage used as a fraction.
- Misread questions — "find the sum" answered as "find the term"; "not" missed; the wrong quantity given.
- Arithmetic under pressure — 7 × 8 = 54, 15 − 8 = 6, √49 = 8.
- Stopping one step early — solving for x when the question asked for 2x + 1, or giving both roots when only the positive one makes sense.
Find which ones are yours
Take your last three tests. For every mark lost that was not a knowledge gap, write which of the six it was. Once it is written down, the pattern is usually obvious: a couple of causes account for most of the losses. Those two are the ones to train; ignore the rest for now.
The five-second checks
Each cause has a check that costs seconds and catches the error before it leaves the page.
- Signs: when a minus sits in front of a bracket, write the expanded line with every term's sign visible before simplifying. When moving a term, say "changes sign" as you write it.
- Copying: after writing the "given" line, look back at the question once, digit by digit. After every simplification, glance up at the line above.
- Units: convert everything in the "given" line, before any formula. Write the unit on the final answer, then ask "is this the unit the question wants?"
- Misreads: underline the quantity asked for and the condition words (not, only, positive, integer) before starting.
- Arithmetic: do the multiplication or subtraction a second way — 7 × 8 as 7 × 10 − 7 × 2; 15 − 8 as "8 + ? = 15".
- Last step: re-read the question's final sentence after you finish, and match it to your final line.
Check by a different route, not by re-reading
Re-reading your own working finds almost nothing, because you read what you intended. Instead:
- Substitute back: put x = 3 into the original equation; does it hold?
- Estimate: a car cannot travel at 4000 km/h; a probability cannot be 1.3; a length cannot be negative.
- Reverse one step: integrate → differentiate to check; factorise → expand to check.
- Count: three roots asked for, three given?
A two-week plan
Days 1–3: classify your last three tests as above. Choose your two top causes.
Days 4–10: on every practice question, do the two checks for your two causes, out loud if you are alone. It will feel slow for two days and then it will not.
Days 11–14: a timed practice set each day. Count the silly mistakes. They should be at most a third of what they were, and the ones that remain will point to the next cause to train.
The mindset
Do not aim to "be more careful". Careful is not an action. Aim to do the sign check and the units check on every question — those are actions, they take five seconds, and they are what careful people actually do.