Colour–word interference · 40 scored trials
The Stroop Machine
The word RED printed in blue. Your job is the colour, not the word — and the word will answer before you do. Forty trials later you get the exact cost of overriding it, in milliseconds.
The test
Model 1935 · interference unit Standby
Answer the ink, not the word
Words will appear one at a time. Press the button matching how the word is printed, ignoring what it says. Forty scored trials in two blocks, plus eight to warm up. Roughly two minutes.
The buttons never move, and the number keys 1 to 4 map onto them from left to right. Answer fast, but answer correctly — mistakes are thrown out of the timing.
Practice —
Your interference cost
- Accuracy
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- Errors
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- Trials used
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- First block
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About This Machine
The Stroop Machine puts you in a small, losing fight with your own reading. A colour word appears in an ink that contradicts it, and you must report the ink. You know the rule perfectly. You will still be slower, every time, and the machine tells you by exactly how much.
The reason is that reading is not something a literate adult chooses to do. It is automatic, in the strict sense: given a word in view, the meaning arrives whether or not it was wanted. So on an incongruent trial two answers reach the finish line together, the wrong one usually first, and you have to suppress it before responding. The delay is the sound of that suppression happening.
What comes out is a single number: your mean time on incongruent trials minus your mean on congruent ones. On button-press versions like this one, most healthy adults land somewhere between 50 and 100 milliseconds. It is one of the most reliably reproduced findings in all of psychology, and one of the very few you can feel arriving.
How It Is Built
Three decisions here are worth explaining, because they are what separate a real measurement from a toy that flatters you.
- The block order is randomised. People speed up with practice, so always running the easy block first would quietly shrink everyone's interference score. The machine tosses a coin and tells you afterwards which you got first.
- The answer buttons carry no words. Labelling them would smuggle a second reading task into the moment of response. They are plain swatches, they never move, and the keys 1 to 4 map onto them left to right.
- Errors are excluded from the timing, as they are in the laboratory, and so are responses under 200 milliseconds or over three seconds — the first is a guess that arrived before you could have seen anything, the second is your attention having gone somewhere else. Both are reported separately rather than silently dropped.
If You Are Colourblind
Then most versions of this test on the internet are useless to you, which is a shame, because the effect has nothing to do with colour. It is about reading being automatic. Colour is just the most convenient thing to put reading in conflict with.
So there are two other ways in. The colourblind-safe palette uses blue, orange, yellow and white, which stay separable under the common red-green deficiencies and differ in lightness as well as hue. The shape mode removes colour altogether: the word CIRCLE appears inside a triangle, and you answer the shape. That version works for monochromats, and it produces the same interference, because the thing being overridden was never the colour.
Why This Exists
In 1935 John Ridley Stroop published a paper called Studies of Interference in Serial Verbal Reactions. He timed people reading cards. Naming the ink colours of a hundred neutral items took his participants an average of 63 seconds; naming the ink colours of a hundred conflicting colour words took 110 seconds. A 74 per cent penalty, from nothing more than the words being readable.
Ninety years on it is the most cited paper in the history of experimental psychology and the effect has resisted every attempt to train it away. You cannot practise your way out of it, you cannot decide not to read, and knowing all of this in advance buys you nothing at all — which you are about to demonstrate to yourself, having just read a full explanation of the trick.
The wider point is what the number represents. Interference is the cost of holding an instruction against a habit, and habits do not care whether they are wanted. That is worth having a personal figure for, even a rough one from a web page on a Tuesday afternoon.
Frequently Asked Questions
What is the Stroop effect?
It is the delay you suffer when you try to name the colour a word is printed in and the word itself names a different colour. Reading is so thoroughly automatic in a literate adult that it runs whether you want it to or not, so the word answers before you do and you have to override it. That override takes measurable time, and this page measures yours.
What counts as a normal interference score?
On button-press versions like this one, healthy adults typically come out somewhere between 50 and 100 milliseconds slower on incongruent trials than on congruent ones. Anything in that region is unremarkable. A single run on a web page is noisy, so treat a very high or very low number as a curiosity rather than a finding about you.
I am colourblind. Can I still take it?
Yes, in two ways. There is a colourblind-safe palette built from blue, orange, yellow and white, which stays distinguishable under the common forms of red-green colour vision deficiency and differs in lightness as well as hue. There is also a shape mode that removes colour from the task entirely: a word appears inside a shape and you respond to the shape. The interference is the same phenomenon, since it comes from reading being automatic rather than from anything about colour.
Why are the answer buttons not labelled with words?
Because labelling them would add a second reading task at the moment of response and contaminate the measurement. The buttons are plain swatches or shapes, they never move, and the number keys 1 to 4 map onto them from left to right, so answering costs you nothing but the decision itself.
Why is the block order random?
Because people get faster with practice, so always running the easy block first would shrink the interference score artificially. The machine tosses a coin each session and tells you afterwards which block you were given first. Stroop himself used separate blocks in 1935, and this keeps that design while removing the order effect.
Does it work on mobile?
Yes. The four response buttons are large and fixed in place, so a thumb works as well as a keyboard. Nothing is saved or uploaded. Reaction times on a touchscreen run a little longer than on a keyboard, but the interference score is a difference between two conditions measured the same way, so it survives that.
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