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The Stroop Effect 1

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Department of Psychological Sciences, School of Science, Birkbeck, University of London

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Module Introduction to Research Methods

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ABIGAIL FERRARI

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The Stroop Effect 3

Interfering Colours: An Experimental Investigation into the Stroop Interference Effect

Abigail Ferrari

Birkbeck, University of London

MSc Psychology

RM01 Research Methods

The Stroop Effect 4

Abstract

John Ridley Stroop was interested in the interference of attention that took place in word reading

tasks. His famous study, the Stroop interference effect has been replicated for a further insight

into the automatic process of word reading over naming colours. Participants (N=69) were asked

to read the colour of words from incongruent and congruent colour conditions and their

responses were timed. There was a significant difference in the time participants took (x =10.86

seconds) to name the incongruent stimulus list (x =31.37 seconds) compared to the congruent

list (x =20.51 seconds).

Interfering Colours: Stroop Interference Test

The Stroop Effect 5

Everyone knows what attention is. It is the taking possession by the mind, in clear and

vivid form, of one out of what seem several simultaneously possible objects or trains of

thought. Focalization, concentration, of consciousness are of its essence. It implies

withdrawal from some things in order to deal effectively with others (James, 1890,

pp.403-4)

Attention has been a fundamental area for both cognitive and experimental Psychology,

and will always remain as one of the more fascinating areas due to the numerous attentional

phenomena that still remain unexplained. Attention is the unique skill of being able to select and

attend to one part of our environment, or as Moray (1969) pointed out it can be referred to as the

“ability to select part of the incoming stimulation for further processing.” (Levitin, 2002, p.363),

However, as noted by several experiments within attention, there are performance limitations

that do not always allow stimuli to be broken down as effectively as one would think. Our

cognitive processes are highly developed and humans have the ability to divide their attention

between more than one task. “In 1886, Cattell described some processing situations as being

automatic, running off essentially without attention” (MacLeod, 1991, p.163) which led to the

distinction of automatic processing. An example of this may be the ability to drive a car and hold

a conversation at the same time, or listening to music as well as reading a book. Automatic

processing is a “type of mental operation that is normally rapid, does not require conscious

awareness, does not interfere with other mental activities and is usually a result of prolonged

practice.” (Gross, 1997, p. 201) However, from certain studies on divided attention using

automatic processing, it appears that it can interfere with other mental processes. Furthermore, if

The Stroop Effect 6

attention is seen as the ability to select part of the environment what happens when other parts of

the environment, or the whole of it, interfere with this selection?

The Stroop test successfully demonstrates the answer to this. John Ridley Stroop (1935)

was concerned with the interference between attentional processes and created what is known as

the Stroop interference effect. It measures the rate of interference in a word-reading task when it

purposefully presents colour words in a conflicting colour ink and requires the participant to

name as rapidly as possible the colour in which the words are printed. For example, if the word

YELLOW is printed in the colour brown, then the participant would need to say the colour the

word is printed in, not the word. The correct answer would be brown. Stroop carried out three

separate experiments. In experiment 1 he used five words and their matching ink colour of red,

green, brown, blue, and purple. For the experimental condition, the words were incongruent;

they were in a different ink colour to their word colour, and in the controlled condition the

colour words were in black ink. Participants had to read the words out loud as quickly as they

could. In experiment 2, the task was to name the colours out loud, however the words were

replaced with solid colour squares. The experimental cards and procedure were identical to the

first experiment but it took participants twice the time to create a response. At first this seems a

simple task, but naming speed is in fact slowed when the colour words are in conflicting colour

fonts; results which are displayed in Stroop’s classic experiment which saw participants taking

on average 47 seconds longer to name ink colours. In his third experiment, Stroop tested his

participants over a period of 8 days using the same task and stimulus as experiment 1 and 2.

Colour-naming times decreased, which could be explained by practice or association.

After his various tests, Stroop accepted Peterson et al. (1925) explanation that reading

words result in one single response whereas colours can result in many responses and therefore

naming colours is slower than reading words. We read words everyday and this has become an

The Stroop Effect 7

unconscious skill that does not need to be thought of. Therefore, it is interesting to see how

automatic reading actually is when asked to ignore the word and only pay attention to another

stimulus, in this case the colour of the word.

These studies have been replicated ten-fold and the results are consistent; Garrett and

Lemmon (1924) identified in their study that colour naming took longer and that this was due to

an interference factor although they could not specify what the interference was. MacLeod’s

(1991) replicas of the Stroop test also had the same results as the original. The test is a powerful

example of the strong automatic nature of reading words and how this can interfere with other

stimuli. “This is because, in the case of words and letters, the association between the idea and

name has taken place so often that the process has become automatic, whereas in the case of

colors and pictures we must by a voluntary effort choose the name” (Cattell, 1886, p.65)

The aim of this investigation is to replicate Stroop’s test and see whether the results

correspond to show interference as a result of automatic word reading. This will be done by

asking participants to name the colour of the word from only two separate lists, both containing

colour words printed in either congruent or incongruent colours. My experimental hypothesis is

uni-directional; the length of time for participants to name the incongruent list will be

significantly greater than the time recorded for the congruent list. Therefore, my null hypothesis

is that there will be no significant difference in the length of time taken to read the congruent

word list and the incongruent word list.

The Stroop Effect 8

Method

Participants

Sixty-nine participants (N = 69) completed both conditions of this study: gender was not

disclosed. Ages ranged from 18 to 47 years, however seven participants did not disclose their

age (x = 29.03, SD = 8.497 years). Participants were all Psychology students at Birkbeck,

University of London who took part as part of their Research Methods module. There were

sixty-nine (N = 69) experimenters who were paired with the participant on a one-to-one basis to

lead the experiment.

Design

A within-subjects design was used, also known as repeated measures because

participants had to be exposed to both conditions of the experiment (congruent and incongruent).

The measure of a participant’s correct/incorrect answers and time taken were repeated under the

conditions of the Independent Variable. The IV was the difference of word colour in the

incongruent colour words as a stimulus to promote interference. There were two dependent

measures that were analysed: the participant’s number of correct answers and the time it took to

complete list A and list B.

Materials

The experimenter had a response sheet (Appendix A) that asked for the participant’s

name, age and whether consent had been given. It also contained a table to note down the

response time and number of incorrect answers for both stimulus A and stimulus B. There were

The Stroop Effect 9

two lists of words on two separate sheets, one for Stimulus A (Appendix B) which contained the

congruent words and the other for stimulus B (Appendix C) containing the incongruent words.

These lists each contained 30 words, spread into two columns of 15 words. The lists of words

were not in the same order for both stimuli, they were randomised. An answer sheet was

prepared for the experimenter with the correct answers (Appendix D). A stopwatch was also

needed in order to get accurate timings. The experimenters were also provided with a procedure

handout of how to conduct the experiment accurately (Appendix E). An instruction handout was

prepared for the participants (Appendix F).

Procedure

Ethical approval has been given for this experiment. Participants were given a reasonable

explanation of the experiment in their lecture theatre and were informed of what the experiment

involved and how it would be carried out. Once explained, participants made their way to the

room where the experiment would take place.

Participants were tested in a room with other participants around them completing the same

experiment. They were asked to sit next to their experimenter and await instruction. On the table

was the response sheet, both stimulus lists and the answers for the experimenter (Appendices A-

D) face down. The stimulus sheets were kept face down until the participant was ready to start

the test. The experimenter asked the participant for their name and age and if they gave informed

consent to take part in the experiment and for their information to be used. It was made very

clear that they had the right to withdraw from the experiment at any time, and to withdraw their

results after they had participated in the experiment. Participants had already been informed of

the experiment in the lecture theatre and so the experimenter explained no further information on

The Stroop Effect 10

the task to them. However, the experimenter reminded the participant to read the words from top

to bottom and left column first.

Once the participant was ready, the experimenter turned over Stimulus A (the congruent words)

and as the participant began to read the first word out loud, the experimenter started the

stopwatch. The experimenter stopped timing the participant after they had said the last word on

the list out loud. As the participant was reading the words, the experimenter was reading the

answer sheet given in order to easily identify any mistakes made. The time (in seconds) was then

noted down on the response sheet along with the number of mistakes that had been made, if any.

The same procedure was completed for stimulus B (the incongruent words). Again, the

experimenter started timing the participant as they said the first word on the list and finished

timing when the last word was said out loud. The experimenter was also reading the answers on

the sheet provided to track any incorrect answers. The time was noted down on the response

sheet, along with the number of incorrect answers should the participant have got any wrong.

Once the participant had completed reading out loud and the experimenter had noted down the

response times and incorrect answers, the experiment was over and the participant could leave

the room.


Results

Inferential Statistics

In Table 1, the table of summary statistics, the overall means for both the congruent and

incongruent stimulus was calculated, along with the median, standard error of the means, and the

standard deviation. This is so I can fully understand my data and analyse it accordingly.

Table 1: Time taken to name colour for congruent and incongruent condition

Congruent condition

(list A): Reaction time

in seconds

Non-congruent

condition (list B):

Reaction time in

seconds

N Valid 69 69

Missing 0 0

Mean 20.5155 31.3788

Std. Error of Mean .95188 1.19631

Median 19.0000 30.0000

Std. Deviation 7.90688 9.93733


Following the results the following histograms were created to show the difference in time it

took participants to name the colour of the words.


As can be seen from Figure 2 above, participants took longer to name the incongruent word list

than the congruent word list, which can be seen in Figure 1. From looking at Figure 1 and 2, I

can see that my data from the congruent condition is positively skewed, and that there is a

somewhat normal distribution of date in the congruent condition.

Participants took 10.86 seconds longer to name the colour words from the incongruent list (x =

31.37, σ x = 1.19) than the congruent list (x = 20.51, σ x = .95)

I also calculated the median because it is the middle of the distribution of the data I collected and

is therefore less sensitive to extreme scores. Again, it shows that the time taken to name the

colour of the words on the incongruent list (M = 30 seconds) is longer than the time taken for the

congruent list (M = 19 seconds). For the congruent list, the median is significantly lower by an

11 second difference. It is evident from both the mean and the median values that it took

participants longer to identify colours of words that were in incongruent ink.

Descriptive Statistics

A dependent paired samples t-Test was carried out as I believe my data to be of a normal

distribution, and as I had expected and therefore used a parametric statistical test. A dependent

paired samples t-Test is effective to use for experiments in which the same participants have

been used in both experimental conditions and where the data is either interval or ratio. I plan to

use the t-Test to compare the two means of the two conditions; the time taken to name the

congruent colour words in stimulus A against the time taken to name the incongruent colour

words of stimulus B. It also means I can look at the correlation between the two experimental

conditions and judge how effective this experiment was.


The paired samples t-Test (Appendix G) showed that participants provided a quicker response

for stimulus A; the congruent words. 10.045(68) = t, p¿ 0.0005. My t-value is greater than the

critical value of t at 3.460, therefore my t-value is significant. Due to the means of the two

stimulus responses and the direction of the t-value, I can conclude that there was a statistically

significant difference in the time it took participants to name the colours of the incongruent list

from 31.38 seconds (secs) ± 9.94 secs compared to the congruent list 20.52 secs ± 7.91 secs (p

¿ 0.05); a difference of 10.86 secs ± 8.98 seconds.

Another useful reason for using the paired samples t-Test is that you can find out if your

experiment has been effective. Although the experiment has delivered a significant test result, it

is useful to know if the effect of the experiment can be measured as important or meaningful. I

therefore calculated Pearson’s correlation coefficient (r). The t-Test had also generated this

automatically. According to Cohen (1988, 1992) if r = 0.50 then the effect accounts for 25% of

the variance. For this experiment r = .59 and therefore the test has been effective, as well as

generating significant results.

After looking at the reaction times for both stimuli, I decided to look at the data according to the

age of the participants, to see if there were certain limitations on timing according to age. At this

point I changed the age variable to categorical and split the ages into 3 separate categories; 18-

24, 25-34, and 35-47. This way I could break down my data far easier than having 69 different

ages on a graph. I did not want my sampling to be skewed, therefore 18-24 has a shorter range of

years compared to 25-34 and 35-47 because more participants were at the beginning of the age

scale and I did not want top heavy data. Graph 1 and 2 below show the separate breakdown for

each stimulus.


Looking at Graph 1 and 2, the most incorrect responses were given for the incongruent stimulus.

However, within the congruent stimulus, the age category of 18-24 gave the most incorrect

responses. The data is not particularly clear in order to judge them against the experiment itself

so I decided to look at the incorrect responses against reaction time for each condition (Graphs 3

and 4) to obtain a clearer picture of what happened within each condition of the experiment.

Within the congruent condition, the higher age categories took longer to name the colour words

but made less mistakes than the 18-24 category who, although were faster to give a response to

the stimulus, made on average more mistakes. Looking at Graph 4, 35-47 and 18-24 gave the

same number of incorrect responses but 35-47 took longer to name the colour words than the


other two age categories. The overall data to compare the two conditions can be found in Graph

5 and 6.


Discussion


The aim of this investigation was to replicate Stroop’s test and observe if automatic word

reading interfered in naming colours.

The findings from the Stroop effect do have a great significance and show the implications of

interference between two cognitive processes. The discovery that the autonomic nature of

reading tasks can interfere with other controlled processes is significant and can be carried over

to other tasks, such as talking on the phone and driving at the same time. The findings of this

experiment concur with Stroop’s results and the replications of his experiment thereafter. There

is a distinct delay in naming the colour of incongruent words, whether the controlled list is in

colour or black ink.

However, as Eysenck (1982) points out, most research and experiments have been concerned

with attentional processes to the external environment. “Experimenters can identify and control

the stimuli presented in the external environment in a way that is simply not possible with

internal determinants of attention.” (Levitin, 2002, p.96)

There are a number of limitations to the experiment I performed. Firstly, the participants were

given an opportunity to practice the Stroop test before entering the official experiment. Looking

at Stroop’s third test, he was able to claim that after practice his participants made less errors and

had a quicker response time albeit over 8 days. However, should the participants of this study

walked into the experiment with no previous knowledge or experience of the Stroop test the

results could be less biased.

Timing is also an issue as the experimenters had to start and stop the stopwatches when the

participant had said their first and last word accordingly – there is always room for human error


and the timings could be a few seconds out for each participant. However, this would have been

the same for both conditions so even taking into account human error, the incongruent condition

would still have taken longer to respond and the difference in time between list A and list B

would be relative to the delay for both stimulus.

The environment was not suitable for this study, in future replicas of the Stroop effect test,

participants should be in a room with the experimenter on their own. In this study, multiple

participants were in one room, therefore whilst the participant is reading the list they can hear

others around them also reading colour words out loud. This is an unwanted added stimulus to

the experiment. The experiment also lacks ecological validity as Eysenck points out that it does

not relate to an internal attentional environment; the participants would not usually find

themselves having to separate the colour ink from the word printed in a real-life setting.

Future studies of the Stroop effect could involve more stimulus lists, similar to the original,

using both colour and other object words. An example of this could be; List A contains colour

words in black ink, List B should involve words such as CAT, TREE, CAR etc. in any colour

ink, then List C should involve the class incongruent colour words. This way it tests how

automatic reading is when asked to read the word or colour. It would test divided attention,

particularly if List A is to name the word and not the colour, and then it is swapped to naming

the colours as a divide in attention has suddenly been created. It also helps research into the

automatic nature of reading when it is an object.

Conclusion

This aim to replicate and observe an interference affect within this experiment was achieved.

The results gained support my experimental hypothesis that participants would take longer to

name the colours of incongruent words than congruent. Therefore, I can reject my null


hypothesis that there would be no significant difference in time between the two stimuli.

Participants took longer to respond to the incongruent stimulus, which means an interference

effect was present between the automatic word reading and naming colours, which was

suggested by Stroop (1935) and Peterson et al. (1925).


References

Cardwell, M., Clark, L., & Meldrum, C. (2004). Psychology for A2 Level. (3rd ed.). London: Harper Collins Publishers Limited.

Cattell, J. M. (1886). The time it takes to see and name objects. Mind, 11, 63-65.

Eysenck, M.W. & Keane, M.T. (1995). Cognitive Psychology: A Student’s Handbook (1st ed.), Hove, UK: Lawrence Erlbaum Associates.

D. J., Levitin (2002). Foundations of Cognitive Psychology: Core readings. Cambridge, MA: MIT Press.

Gleitman, H., Gross, J., & Reisberg, D. (2010). Psychology. (8th ed.). London, UK: W.W.Norton & Co..

Gross, R. (1997). Psychology: The Science of Mind and Behaviour. (3rd ed.). London: Psychology Press.

Hayes, N. (1994). Foundations of Psychology: An Introductory Text. London: Routledge.

James, W. (1890). The Principles of Psychology. New York: Henry Holt, Vol. 1, pp. 403-404.

MacLeod, C. M., & MacDonald, P. M. (2000). Interdimensional interference in the Stroop effect: uncovering the cognitive and neural anatomy of attention. Trends in Cognitive Sciences, 4(10), 383-391.

MacLeod, C. M. (1991). Half a Century of Research on the Stroop Effect: An Integrative Review. Psychological Bulletin, 109(2), 163-203.


Appendix A

Response sheet

This sheet is to be completed by the experimenter and handed back to the demonstrator at the end of the test

Name of participant

Age of participant Consent given Stimulus A: Congruent Stimulus B: Incongruent

Response time (seconds)

Number of incorrect answers

Response time (seconds)

Number of incorrect answers

Name of experimenter: ________________________________________


Appendix B


Appendix C


Appendix D


Appendix E

Procedure

The list of words should not be visible to the participant until you are ready to start

The experimenter (of the pair), will present the participant with the first stimulus set (A or B)

As soon as the participant has been presented with the words by the experimenter, he/she is to read the list out aloud to the experimenter

Start the time when the participant reads out the first word. Stop the timer when the participant reads out the last word. Record the time (in seconds) on the record sheet provided

Count the number of incorrect responses made. Record the number


Appendix F

Instructions for participants

In this experiment you are required to name the colors of the ink for each word

Read the list of words from top to bottom, left column first

Name the ink colours as fast and as accurately as you can

Your first response per word will be taken

If you make a mistake move onto the next word


Appendix G

Paired Samples Statistics

Mean N Std. Deviation Std. Error Mean

Pair 1 Non-congruent condition (list B): Reaction time in seconds

31.3788 69 9.93733 1.19631

Congruent condition (list A): Reaction time in seconds

20.5155 69 7.90688 .95188

Paired Samples Correlations

N Correlation Sig.

Pair 1 Non-congruent condition (list B): Reaction time in seconds & Congruent condition (list A): Reaction time in seconds

69 .513 .000

Paired Samples Test

Paired Differences

t dfSig. (2-tailed)Mean

Std. Deviation

Std. Error Mean

95% Confidence Interval of the Difference

Lower Upper

Pair 1 Non-congruent condition (list B): Reaction time in seconds - Congruent condition (list A): Reaction time in seconds

10.86333 8.98310 1.08144 8.70536 13.02131 10.045 68 .000

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