Lab 1: The Stroop Effect (1935)
Measuring interference effects
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This course aims to equip you with;
◦ an understanding of some basic studies that have
been useful in Psychology
◦ an understanding of basic software tools often used
to run and report those studies:
 Excel, for basic data handling
 SPSS for statistical analysis
 PsychoPy for presenting stimuli and collecting data
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Each semester you will have 5 x 3hr lab classes
You will be required to write 3 reports for each
semester (a simulated journal article) on which you
will be assessed
It is essential that you come to the lab class at your
allotted time;
◦ if you miss a lab class you cannot just go to a
different session and use the data from there,
because the data they collect may differ from that in
your group
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In this class you will see a simple classic psychology
experiment replicated using modern technology.
You will learn about;
◦ the computers in these labs
◦ why we should run experiments at all
◦ conducting a literature search
◦ the basic components of PsychoPy
◦ conducting simple analyses in Excel
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All materials will be made available via WebCT,
including;
◦ slides from the lectures
◦ the experiments that were run
◦ data files to analyse
You should log on to WebCT as soon as possible to
make sure that you have access to these materials
http://webct.nottingham.ac.uk
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Do not store any files in My Documents or in your
Desktop folder. On these computers anything stored
there will be lost. Use you H: folder (see below)
In addition to the C: drive (the local computer) you will
see some additional (network) disk drives listed as:
◦ H:<username> - this is for you to read and write files and
will remain the same from any computer you log in to.
Other users do not have access to this folder
◦ Practicals – this contains materials used by some lecturers.
They may ask you to fetch things from here, but you can’t
save files here
◦ Class Share – this disk allows you to save files that anyone
else can read/change/delete
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There was a time when people tried to understand
behaviour by introspection; by thinking really hard
about why we do what we do
When someone has a theory based on their own
behaviour, how do we know that they are right?
We need to collect data to help us understand
ourselves and the world we live in
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Sometimes we can collect data by measuring things
in the ‘real world’
That is better than just ‘thinking’ because we have
more information
But in the ‘real world’ many things vary, in addition
to the thing you are studying
How do you know whether it was your variable of
interest that caused the change of behaviour you
measured?
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In experiments we aim to let very few things vary
That helps us, when we measure a change, to narrow
down its cause
Note: Possibly, when fewer things vary, the less like the ‘real world’ our
experiment becomes? Is it better to know something about an artificial
situation, or to have a good guess about something realistic?
We want to keep things simple, but hopefully they
will generalise to more complex situations
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In 1935 John Ridley Stroop published a series of
experiments about a new effect
◦ Stroop, John Ridley (1935). "Studies of interference in serial
verbal reactions". Journal of Experimental Psychology 18:
643–662.
In this paper he described an effect of mental
processing that is extremely robust and has
become very well known; the Stroop Effect
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The paper contains several experiments including
this one (Exp 2):
◦ participants had to say out loud the colour of letters
for a series of 100 words printed on some cards
◦ but the letters were also spelling out names of other
colours (the name and the colour were incongruent)
◦ for comparison he also measured the time taken to
call out the colour of 100 squares
◦ subjects took longer to call out letter colours than
colours of blocks
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Maybe subjects were faster with the squares because
there was more red ink on the squares?
An alternative control would be to use
◦ red letters to spell the word red
(word and colour are congruent)
◦ green letters to spell the word red
(word and colour are incongruent)
The Stroop effect has now been using many different
variants of the task
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Read out the colour of the letters in the words below
RED
BLUE
GREEN
YELLOW
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Read out the colour of the letters in the words below
BLUE
YELLOW
RED
GREEN
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We can use computers to do a much more accurate
job of measuring the time it takes participants to
detect the letter colour
Present words drawn in red, green or blue where the
word and the letter colours are congruent or
incongruent
Get subjects to press one of 3 keys depending on
what the letter colours are
It should also be a lot quieter to do it this way than
in Stroop’s experiment!
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Note that this is an experiment because we;
◦ manipulate an independent variable (IV)
◦ measure a dependent variable (DV)
For the Stroop Effect;
◦ the IV is the congruence of the letter colour and word
◦ the DV is the reaction time in reporting the letter
colour
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We need to use software that can measure the
responses and reaction times (RTs) of the subjects
For this we’re going to use PsychoPy
This is free software that you can install on your own
computer (Windows, Mac or Linux), by downloading
it from www.psychopy.org
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When you first run PsychoPy two windows come up;
◦ the Coder view allows you to run scripts and do
simple programming in a language called Python
◦ the Builder view allows you to create experiments
visually and then runs them
◦ we will be using the Builder view most of the time, so
close the Coder view for now (you can get it back from
the >View menu if you need to)
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Let’s try it for real
◦ save the Stroop experiment to your <H:> folder. Make sure
you know where you saved it!
◦ open it in PsychoPy




go to the Builder view, then >File>Open…
navigate to where you just saved the exp
open it
(alternatively double click the file stroop.psyexp)
Run the experiment
◦ either press the green button with the running man
◦ or press Ctrl-R
When asked for the participant ID, type in your login
name
(e.g. lpyxxxx)
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