When you get to this page, you should have completed the Stroop experiment and have your mean reaction times for the first and second blocks. If not, you will need to re-do the experiments.

In the first block, the words and the colors were identical, so you could produce the correct answer two different ways. Usually people find this an easy task, where they are fast and make few errors.

In the second block, the words and colors were mismatched, leading to interference. Because reading is an automatic process, the words interfere with your ability to identify the colors. This makes it a hard task, and people often make errors.

If you subtract the block one reaction time from the block two reaction time, that provides some indication of the magnitude of the Stroop effect. Our brief experiment can only give an approximate value of this effect because the two-block design confounds condition with order. People often speed up as they practice a task, so the second block has a natural advantage over the first. Psychologists often employ a counterbalanced design to minimize the impact of practice in experiments like the one we just conducted. In our case, the estimate you found for the magnitude of the Stroop effect is likely to be a little smaller than it should be. Because the effect is so large, it is likely to be present anyway.

Your estimate may also be too small if you made more errors in the interference condition than in the matching condition.

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