To investigate the stroop effect

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The purpose of this experiment was to investigate the Stroop effect, and the difference in reaction time between the congruent and incongruent conditions. There were 21 participants involved. A repeated measures design was used for the two conditions: congruent and incongruent. Participants were required to look at coloured words and name the ink colour, whilst ignoring the actual word. The results show that reaction time was faster during the congruent condition and fewer errors were made. The level of significance was P<0.001, meaning that the probability of the results being due to chance is less than 1%. This proved both hypotheses to be correct, thus supporting the original Stroop investigation (1935).


The process of 'attention' requires the cognitive ability of an individual to selectively concentrate on a certain stimuli in the environment whilst selectively ignoring others. However, if attention is diverted by interference from another stimulus. Interference became very influential in cognitive and neural components of selective attention and was the basis for much research prior to Stroop.¹

The Stroop effect was originally named after John Ridley Stroop and was published in 1935. The test demonstrates the difference in reaction time of naming colours, reading names of colour, and naming colours of words printed in different ink. It also aims to measure cognitive ability (learning and memory) and attention focus.² Thus it can be argued that the word and its meaning can be processed even though it is not relevant.

In 'Stroops classic article', he raised two questions: what effect each dimension of the different stimuli would have on naming the other dimension i.e. colour ink and the word itself. Secondly, what effect practice would have on the reaction time to both stimuli. Given this, he conducted two experiments. Experiment 1 investigated the effect of different ink colours on reading the words out aloud. The second experiment focussed on reading the colours aloud. Results showed that participants took approximately 47 seconds longer to name the colours of the incongruent condition as opposed to the coloured squares. Stroop concluded that this 74% increase was due to an interference effect.³

Stroop's research originated from James McKeen Cattell (1886), who found that responding to objects and colours took longer to read aloud than words. The association between the name and idea took place so frequently that it became an automated process. Unlike with pictures and colours, an intentional effort had to be made.4

The Stroop effect was used to discredit the theory of controlled and automatic processing by Schneider and Shiffrin (1977). Their theory concluded that controlled processing was slower than automatic. Also, once a task was automated it could be done with no conscious effort, and this would affect all other activities.5

Sheibe, Shaver and Carrier (1967) found that if the word was congruent, it would be identified quicker than if it was incongruent. This also supports for Stroop's investigation (1935).

However, all prior researchers were interested in studying the interference between conflicting processes and none thought to combine colours and words until Stroop.

Since Stroops original experiment, there have been many variations made to test different phenomena. For example, sorting and matching versions of the colour word task, the picture-word interference task, and the stroop colour-word test.6

The aim of this experiment is to prove Stroops theory, and to investigate how interference affects participants' ability in both conditions

Hypothesis 1:

The reaction time for the congruent condition will be significantly faster than that of the incongruent condition.

Hypothesis 2:

Fewer mistakes will be made within the congruent condition in comparison to the incongruent condition.



A repeated measures design was used for this experiment. This requires using the same participants for each condition - congruent and incongruent. This allows the responses from an individual for the congruent condition to be directly compared to their response for the incongruent condition.7

There were two conditions being compared. The congruent condition consisted of words written in the same colour ink e.g. the word 'red' was written in red ink. The second condition was incongruent e.g. 'red' written in blue ink.


A sample of 21 Psychology students from Royal Holloway University of London participated, aged between 18-21; 16 females and 5 males. However, 12 were excluded due to there being no response, or their responses were excluded.


A computer was used to generate the two stimuli and collect the participants' response. The program used was 'DMDX' and involved participants to respond to the ink colour of written words, whilst ignoring the word. The words were displayed in the centre of the screen for approximately 750ms. Before each word, a fixation point (+) was presented in the centre of the screen before the next word. The interval between the fixation point and the next word also lasted 750ms. There was also an 'inter-stimulus interval' (ISI) of 2000ms between fixation and word presentation.


The procedure of the experiment was followed in two parts. During part 1, participants were asked to follow instructions and confirm their gender. This allowed information to be gathered about the participants so that analysis could be carried out later about whether or not the sample used was bias. In Part 2, the participants were given 9 trial runs, 4 congruent and 5 incongruent. Participants were then asked to view the fixation point (+) which was followed by words displayed in different coloured ink and were asked to indicate the ink colour. The following keys were used to indicate the chosen colour: 1 - red, 2 - green, 3 - blue, 4 - yellow. In total the participants were asked to carry out 200 trials for 5 minutes. Each condition consisted of 100 trials - 25 of each colour. The responses from each participant for each condition were generated into a database.


Participants: 5 males and 16 females. Total: 21

Descriptives: mean age: 20.14 (6)

Descriptives: Table

The results table show that the mean reaction time and (standard deviation) for the congruent condition was 732 milliseconds (189) and 860ms (232) for the incongruent condition. This shows that the reaction time for the congruent was faster than the incongruent condition, thus accepting the alternate hypothesis. The mean error and (SD) for the congruent condition was 5 (3) and 10 (6) for the incongruent condition.


RTs t-test (p value): t(20) = 8.34, p = <.001

Errors t-test (p value): t (20) = 4.82, p = <.001

The chosen statistical analysis was a t-test. This allows us to compare the results in terms of reliability as the t value obtained is related to a specific value for the degrees of freedom for the experiment. The calculated t value at the confidence level of p=<.001 is less than the value predicted for the p value. This allows us to reject the null hypothesis and accept the alternate as the results obtained can be said to be 99% concurrent due to the independent variable.


The results from this experiment illustrate that the reaction time for the congruent condition was significantly faster than that of the incongruent condition, as seen in fig 1. After the standard deviation for 'error' was calculated, it can be seen that the incongruent condition had significantly higher error values in comparison to the congruent condition (see fig 2). This supports both hypotheses and therefore proves the Stroop effect (1935), where participants were placed in two different conditions. The participants that were assessed in the congruent condition were found to have significantly lower reaction times when naming the colour of the ink in comparison to those in the incongruent condition. The theory of Sheibe, Shaver and Carrier can also be applied to our results as it has been proven that interference does occur via automatic processing. Their research also proved Stroops work as they found that if the colour word was written with the same colour then the time to state this by the participant was notably faster in comparison to words that used different coloured ink. This then gave way for research to be performed in regards to stimuli response as later research found that there was less interference when the stimuli was similar e.g. the word blueberry is already associated with the colour blue and therefore this would have less interference than for example the word apple.

Limitations and Further research

The main problem encountered was the sample bias. The participants that were used were all psychology students from university, and cannot be generalised to society. Therefore the results gained are unreliable. The experiment can also be heavily criticised for being gender bias, as there were only five males in comparison to sixteen female participants. This also had a confounding affect on the results as shown in fig 1 and 2, as male participants had a faster reaction time than females. Therefore we cannot generalise our findings as the sample used was not equal.

Another limitation was the effect the uncontrolled variables had upon the investigation such as noise and light which acted as interference and could have affected the reaction time and number of errors.

Improvements are required to increase the validity of the results. A larger sample size would make this experiment more reliable and more able to generalise the findings. To improve this aspect further, a wider age range would increase the validity as age could have an affect on attention. This could be used to research further into the mind of individuals. Furthermore, a more diverse gender would be beneficial in increasing the validity of the experiment as there are key differences between male and females as has been indicated throughout the field of psychology. In order to develop this investigation, interference can also be measured aswell as reaction time. This would produce results of a higher accuracy, increasing the validity. However, this would require the external environment to be adapted accordingly, thus eliminating any extraneous variables that could alter the results.


  • Charles Stangor (3rd Ed) Research Methods for the Behavioural Sciences
  • Elizabeth A. Syles (1997). The psychology of attention
  • Jensen, A. R., & Rohwer, W. D., Jr. (1966). The Stroop colour-word test: A review. Acta Psychologica.
  • MacLeod, C.M. (1991) Half a century of research on the stroop effect: an intergrative review. Psychological Bulletin, 109, 163-203
  • MacLeod, C.M. (1991). John Ridley Stroop: creator of a landmark cognitive task. Canadian Journal of Psychology.
  • MacLeod, C. M., & MacDonald, P. A. (2000). Interdimensional interference in the Stroop effect: uncovering the cognitive and neural anatomy of attention. Trends in Cognitive Sciences.
  1. MacLeod, C. M., & MacDonald, P. A. (2000). Interdimensional interference in the Stroop effect: uncovering the cognitive and neural anatomy of attention. Trends in Cognitive Sciences.
  2. MacLeod, C.M. (1991). John Ridley Stroop: creator of a landmark cognitive task. Canadian Journal of Psychology.
  3. - MacLeod, C.M. (1991) Half a century of research on the stroop effect: an integrative review. Psychological bulletin
  4. Jensen, A. R., & Rohwer, W. D., Jr. (1966). The Stroop colour-word test: A review. Acta Psychologica.
  5. Elizabeth A. Syles (1997). The psychology of attention
  6. Charles Stangor (3rd Ed) Research Methods for the Behavioural Sciences