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What Affects Stroop Interference — Practice, Age and Bilingualism
Even for the same person, interference changes with practice, age, the language being read, color vision, task design and how you feel that day. The factors to know before comparing results, with the research behind them.
Even for the same person, Stroop interference can vary quite a bit depending on the conditions. Here are the factors worth knowing before you compare results.
Practice
Ever since Experiment 3 of Stroop's original paper, practice has been shown to reduce interference without eliminating it. In computerized tasks, too, reaction times drop quickly over the first few sessions, and interference shrinks somewhat. So comparing your first session directly with your tenth mixes in practice effects. The 8 practice trials on this site are for learning the button positions and are not recorded.
Age
Children who have not yet learned to read show almost no interference; it becomes clear around the early elementary school years, when reading becomes automatic. Comalli, Wapner & Werner (1962) reported a pattern in which interference is large in children, smallest in young adults, and grows again in old age.
Whether the increase in old age reflects a weakening of inhibition itself or a general slowing of processing speed is debated. A meta-analysis by Verhaeghen & De Meersman (1998) concluded that much of the age-related increase in interference is explained by general slowing. Keep in mind that people who respond more slowly overall also tend to show larger differences in ms.
Bilingualism and reading skill
Interference depends on how automatically you read the letters. When bilinguals are shown color words in both languages, interference is greatest when the language of the word matches the response language; words in the other language also interfere, but generally less (Preston & Lambert, 1969; Dyer, 1971). Interference tends to be larger in the more fluent language.
This is why this site uses each language's own color words for all 11 languages. Testing on a page in a language you are not fluent in can produce smaller interference, so use the same language page when comparing results. The words for each language are listed on the Color words page.
Color vision
With red-green color vision deficiency, telling red and green ink apart takes longer, which can make you slower or cause more errors regardless of interference. The buttons are also labeled with color names, but it remains a task in which you judge by the ink color. In this case, the word-meaning task (reverse Stroop) is relatively less affected.
Task design
- Proportion of congruent trials — The more congruent trials there are, the more people start to rely on the word, and the larger the interference on incongruent trials. This site mixes congruent, incongruent and neutral trials in equal thirds.
- Trial order — Interference tends to be smaller on the trial right after an incongruent one. This site orders trials so the same correct answer never appears twice in a row and the same condition never repeats four or more times in a row.
- Response method — Interference tends to be smaller with button responses than with vocal responses. There may also be differences between keyboard and touch.
- Time limit — A short limit makes people rush and make more errors, which can shift the reaction time difference into an error rate difference.
How you feel that day
Fatigue, lack of sleep, caffeine, background noise and doing something else at the same time all shake up reaction times. To track trends, it is best to measure at a similar time of day in a similar environment.
References
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18(6), 643–662.
Comalli, P. E., Jr., Wapner, S., & Werner, H. (1962). Interference effects of Stroop color-word test in childhood, adulthood, and aging. The Journal of Genetic Psychology, 100(1), 47–53.
Verhaeghen, P., & De Meersman, L. (1998). Aging and the Stroop effect: A meta-analysis. Psychology and Aging, 13(1), 120–126.
Preston, M. S., & Lambert, W. E. (1969). Interlingual interference in a bilingual version of the Stroop color-word task. Journal of Verbal Learning and Verbal Behavior, 8(2), 295–301.
Dyer, F. N. (1971). Color-naming interference in monolinguals and bilinguals. Journal of Verbal Learning and Verbal Behavior, 10(3), 297–302.