The Stroop Effect
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18(6), 643–662. https://doi.org/10.1037/h0054651
Name the color of the ink, not the word: red, blue, green. The word is easy to ignore in principle and hard to ignore in practice. Knowing what the demo is about does not make it go away. Start with the quick version, then try Stroop’s own cards, then measure the effect trial by trial.
Part 1: A quick version
Two short cards of 15 words each. Say the ink color of each word aloud, left to right, as fast as you can. On the first card every word is printed in the color it names; on the second, every word names a different color. The effect is strong enough that you will probably feel it even with only 15 items.
Part 2: Stroop’s cards, read aloud
Each card has 50 items in 5 rows. When you press Start the card turns over and the clock starts. Say each answer aloud, left to right, row by row, as fast as you can. If you make a mistake, correct it before going on. Press Done (or the space bar) as soon as you say the last item. Try all four tasks; the order below is a good one.
Your card times
Your times are scaled from 50 items to 100 so they can be set beside Stroop’s, which are means for 100 items (70 students read the words; 100 students named the colors).
Part 3: Measure it trial by trial
One word appears at a time. Press the key for its ink color: D red, F green, J blue, K yellow, or tap the buttons. Some words name their own ink color (congruent), some name a different color (incongruent), and some are a row of Xs (neutral). Do the practice run first to learn the keys.
What the demo shows
Stroop’s students named the colors of 100 solid squares in 63.3 seconds on average. Naming the ink colors of 100 color words, each printed in a color other than the one it named, took 110.3 seconds, an increase of 74%. Nearly every student was slower on the word card than the group’s average on the squares. The reverse test showed almost nothing: reading the same words aloud took 43.3 seconds when they were printed in conflicting colors and 41.0 seconds when they were printed in black.
The asymmetry is the point. Reading a word is so practiced that it happens whether or not you intend it, and the word’s name competes with the color name you are trying to say. Naming colors is slower and less practiced, so the ink color does little to disturb reading. Stroop showed that the balance can shift: after eight days of practice naming ink colors, his students got faster at it and began to show interference when reading the words.
The trial-by-trial version separates two parts of the effect. Interference is the slowing on incongruent trials compared with neutral ones. Facilitation is the speeding on congruent trials compared with neutral ones. Across many studies interference is much larger than facilitation (MacLeod, 1991). Key presses usually show a smaller effect than naming aloud, because a key press is not a spoken name and competes less directly with the word.
MacLeod, C. M. (1991). Half a century of research on the Stroop effect: An integrative review. Psychological Bulletin, 109(2), 163–203. https://doi.org/10.1037/0033-2909.109.2.163
Your trial-by-trial data
Your data stay in this page only. Nothing is saved; reloading or closing the page erases them. Practice trials are listed but left out of the summary. Mean reaction times use correct answers between 200 ms and 2500 ms.
All trials
How this demo differs from Stroop’s experiments
- The quick version is not one of Stroop’s tests. It compares words printed in their own colors with words printed in other colors; Stroop never used words printed in their own colors, and his baseline for naming was solid squares.
- Stroop’s cards were printed sheets of 100 items in 10 rows, read in daylight, with each student reading two sheets of each test. These cards have 50 items on a screen, and the times are doubled for comparison.
- An experimenter timed Stroop’s students with a stopwatch to a fifth of a second and checked every answer. Here you time yourself, so your own reaction in pressing Done adds a little to every time, and you count your own uncorrected errors. Stroop added twice the average time per item for each uncorrected error; this demo records errors but does not adjust the times.
- Stroop arranged each row so that every color appeared twice, no color followed itself, and no word was printed in the color it named. These cards follow the same rules, with a new random arrangement each time you shuffle.
- The trial-by-trial part is a later form of the task, not one of Stroop’s experiments. It uses four colors that are easy to tell apart on a dark screen, key presses instead of speech, and a row of Xs as the neutral baseline. Stroop had no congruent condition.