The Stroop Test: What It Measures and How to Read Your Score

If you have ever been shown the word "RED" printed in green ink and asked to name the ink color, you have taken the Stroop Test. It looks like a party trick — almost everyone is slower and more error-prone when the word and the ink color conflict than when they match. But the test is one of the most-used instruments in the history of cognitive psychology, and the effect it measures — the Stroop effect — has been replicated thousands of times across cultures, age groups, and clinical populations. This article explains what the test actually measures, how to interpret your score, and what it does and does not tell you about your cognition.

The original 1935 study

John Ridley Stroop, an American psychologist, published the original study in 1935 in the Journal of Experimental Psychology. He observed that naming the ink color of a color word was substantially slower when the word named a different color (incongruent) than when the word named the same color (congruent) or when the stimulus was a colored square (no word at all). The difference in reaction time between incongruent and congruent trials is what we now call the Stroop effect, and it typically falls in the range of 100-300 milliseconds for healthy adults.

Why the effect happens

The standard explanation, supported by decades of research, is that reading is an automatic process for literate adults. When you see a word, your brain retrieves its meaning without any deliberate effort — you can't help but read it. Naming the ink color of a stimulus, by contrast, is a less-practiced, more deliberate process. When the two conflict, the automatic reading response competes with the deliberate color-naming response for control of your output. You have to inhibit the reading response to respond correctly, and that inhibition takes time.

This is why the Stroop Test is taken to measure cognitive control — specifically, the ability to suppress a prepotent (dominant) response. Cognitive control is one of the core components of executive function, the umbrella term for the mental processes that allow us to plan, focus attention, remember instructions, and juggle multiple tasks. Executive function is what lets you stick to a boring task, resist an impulse, or follow a complex instruction without getting distracted.

How to interpret your score

If you took our Stroop Test tool, you got three numbers: your average reaction time on congruent trials, your average on incongruent trials, and the difference (the Stroop effect). Here is what each tells you.

Congruent reaction time

This is mostly a measure of basic processing speed and motor response time. Typical values for healthy adults are 500-700 ms. Faster values indicate faster overall processing; slower values can reflect fatigue, age, medication, or simply the hardware you're using (display latency, input device).

Incongruent reaction time

Typical values are 700-1000 ms. The slower you are here relative to your congruent baseline, the larger your Stroop effect — which is generally interpreted as weaker inhibitory control. But the interpretation is not as simple as "slower is worse." Some people slow down on incongruent trials to maintain accuracy; others speed up but make more errors. Both strategies are normal.

Stroop effect (the difference)

This is the most meaningful single number. Typical adult values are 150-300 ms. Larger effects indicate more interference from the automatic reading response — which is associated, in the research literature, with weaker inhibitory control. The effect tends to be larger in children (whose inhibitory control is still developing), in older adults, and in individuals with conditions including ADHD, schizophrenia, and traumatic brain injury. It tends to be smaller in people who have practiced the task.

What the Stroop Test does not tell you

A single Stroop Test score does not diagnose anything. Many factors influence your score on any given day: fatigue, caffeine, time of day, distractions, the device you're using. A single low score means almost nothing. What is meaningful is your score relative to your own baseline over many sessions, or your score relative to a population norm under standardized conditions administered by a clinician.

If you have a clinical concern — for example, you suspect you or your child has ADHD — the right next step is to consult a licensed psychologist or psychiatrist. They may use the Stroop Test as part of a larger assessment battery, but they will not diagnose based on it alone, and neither will we.

Why the Stroop Test has lasted 90 years

The test's longevity in cognitive psychology comes from three properties. First, the effect is large and reliable — virtually every literate adult shows it, and the effect size is big enough to measure easily. Second, the task cleanly isolates one component of executive function (response inhibition) without requiring complex instructions or expensive equipment. Third, it is sensitive to a wide range of clinical and developmental differences, which makes it useful as a screening instrument across many populations.

A comprehensive review by Colin MacLeod in 1991 (a "review of the Stroop literature on its 50th anniversary") cited over 400 published studies using the task. By now, three decades later, the count is in the thousands. Few cognitive tasks have been so thoroughly studied.

Practice effects

If you take the Stroop Test daily for a week, your incongruent reaction time will probably drop by 50-100 ms, and your Stroop effect will shrink correspondingly. This is a practice effect on the specific task. As with other brain-training tasks, the question of whether this practice generalizes to broader cognitive control is contested. The honest summary: practicing Stroop makes you better at Stroop; whether it makes you better at inhibiting impulses in daily life is less clear.

Sources

• Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18(6), 643-662.
• MacLeod, C. M. (1991). Half a century of research on the Stroop effect: An integrative review. Psychological Bulletin, 109(2), 163-203.
• Scarpina, F., & Tagini, S. (2017). The Stroop Color and Word Test and the Stroop-like effect. Frontiers in Psychology.