Above the 75th percentile
The score here is neither the time nor the accuracy on its own but their ratio: accuracy divided by how fast you answer green. So there is no single way to reach the top of the scale. You can be fast and never break on red; you cannot be fast at the cost of breaking, and you cannot be flawless at the cost of being slow — the score eats both trades. The hard part of the task is built deliberately: seventy per cent of the trials are green, and by the tenth one the hand is pressing on its own. Red after a long run of greens is the actual test. The top band means the habit did form and you stopped it anyway.
Middle: 25–75
Two hundred to two hundred and eighty is the ordinary zone, and behind it there is almost always one break on red rather than general slowness. That is by design: the habit of pressing is built first and tested second. The stimulus lives six hundred milliseconds and the gap between trials is a little over a second, and at that tempo an error comes not from inattention but from a decision made before the stimulus was identified. What moves the score most in this zone is not trying to win the next trial back. The trial right after a mistake is the most vulnerable one, because the tempo is already broken and attention has gone backwards.
Below the 25th percentile
The bottom of this scale is reached by two very different routes, and a single number cannot tell them apart: either there were several presses on red, or the response to green was steady but slow. The first is about inhibition, the second about tempo, and the score adds them together. Before reading the result as impulsivity, subtract the ordinary things: a touchscreen and a 60 Hz panel, noise, hurry, a tab in the background. Twenty trials is a short run even by short-form standards — the literature uses twenty to fifty and we sit at the lower bound. The number is neither a diagnosis nor a ruling-out of one.