The Stroop effect, first described by John Ridley Stroop in 1935, is the delay that happens when the meaning of a word conflicts with the colour it is printed in. Reading is so automatic that your brain must actively suppress the word to name the ink colour — that suppression takes measurable time.

It measures selective attention and inhibitory control — your ability to suppress an automatic response (reading) in favour of a deliberate one (naming the ink colour). Your interference score is the difference between your speed on matching and mismatching trials.

Accuracy of 27 or more out of 30 with an average response under 900 ms is a strong performance. Most people respond noticeably slower on mismatching trials — an interference gap of under 150 ms shows excellent inhibitory control.