Numerical Stroop Task (Size Congruency Effect)
The numerical Stroop effect(Officially, it is called the Size Congruency Effect), a concept rooted in cognitive psychology, refers to the interference that occurs when individuals are asked to compare numerical values or physical sizes of digits presented together.
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FAQ
What is the Numerical Stroop task?
The Numerical Stroop task measures the size congruency effect. You must judge which number is physically larger, ignoring its value.
How does a trial work?
On each trial, two numbers appear. Press A if the left number is physically larger, L if the right number is physically larger.
What feedback do I get?
After each response, you will see feedback indicating if you were correct or not.
Experiment Introduction
The numerical Stroop task is a great and popular task. Officially, it is called the Size Congruency Effect. The effect was first described by Besner and Coltheart (1979). Another nice early study on the effect was by Henik and Tzelgov (1982). There is a good review by Reike and Schwarz (2017).
Example: The Henik and Tzelgov (1982) paper summarizes the effect perfectly:
In this study, subjects were asked to judge which of two digits (e.g., 3 5) was larger either in physical or in numerical size. Reaction times were facilitated when the irrelevant dimension was congruent with the relevant dimension and were inhibited when the two were incongruent (size congruity effect). Although judgments based on physical size were faster, their speed was affected by the numerical distance between the members of the digit pair, indicating that numerical distance is automatically computed even when it is irrelevant to the comparative judgment being required by the task. This finding argues for parallel processing of physical and semantic information in this task.
In short, you are repeatedly shown a pair of numbers. One is physically larger than the other. You need to figure out which one is the physically larger of the pair. If the left one is larger, you press the A key (which is left on the keyboard) and if the right one is larger, you press the L key (which is right under the keyboard).
This can be confusing, because a number can be numerically smaller, yet physically larger. Look at the image below to see some examples.
The difference between how fast you respond in congruent and incongruent trials is the Size Congruency Effect.
The Numerical Stroop Task Vs Original Stroop Task
Similar to the "real" Stroop task, you respond to stimuli that are sometimes containing "confusing information".
Here is a comparison of the two:
| Condition | Congruent (Straightforward) | Incongruent (Confusing) |
|---|---|---|
| Numerical Stroop | 48 | 93 |
| Original Stroop | GREEN | BLUE |
In the **Original Stroop** task, the color word "GREEN" and "BLUE" represent congruent and incongruent conditions respectively.
| Numerical Stroop | Original Stroop | |
|---|---|---|
| Stimuli | Numbers | Colour Words |
| Task | Look which number is physically larger | Look what the ink color of a word is |
| Congruent | The smaller number is lower in value than higher value | The ink color and word meaning are the same |
| Incongruent | The smaller number is higher in value than smaller number | The ink color and word meaning do not match |
Further reading
- Besner, D., & Coltheart, M. (1979). Ideographic and alphabetic processing in skilled reading of English. Neuropsychologia, 17, 467–472.
- Bowman, K.A. (2020). Response Time modeling for the size congruity effect: Early vs. late interaction. Link
- Henik, A., & Tzelgov, J. (1982). Is three greater than five: The relation between physical and semantic size in comparison tasks. Memory & Cognition, 10, 389–395. Link
- Reike, D., Schwarz, W. Exploring the origin of the number-size congruency effect: Sensitivity or response bias?. Attention, Perception, & Psychophysics, 79, 383–388 (2017).