The reaction time test: go on green, hold on red

A circle turns green, or a lamp flashes, or a tone sounds, and you respond. Then a red one appears, or a pale flicker, and you are supposed to keep still. That is the reaction test.

People train the first half and skip the second. That's how you get fast at the wrong moments.

Go / no-go is the point

A driver who's slow to a genuine signal is a problem. A driver who treats every flicker as a genuine signal is also a problem. The test is built to catch both. If you only watch your best time, you're watching the wrong number.

Some batteries use a visual flash. Some add sound. Some want you to ignore near-misses that look almost like the target. Read the live instructions. The family is the same: wait, go, withhold.

How people talk themselves into errors

• Anticipation. You click because it “must be soon”. It wasn't.
• Clicking again after a miss, as if to make up for it. The next trial is a new trial.
• Tight shoulders and a hovering finger. If you're already halfway to the click, you will click red as well.

Mixed trials, same hand

Practise mixed trials, not a run of pure greens. Watch the clicks you should have held as closely as the ones you missed. Sit as you will sit on the day: same hand, phone away, no soundtrack. If the score looks good because you never saw a red, the score is lying.