1. Lab 1: The Stroop Effect (1935) Measuring interference effects
First year practicals
Lab 1: The Stroop Effect (1935) Measuring interference effects

2. Course objectives 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

3. The course 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

4. Objectives of this class
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

5. Practicalities - course materials
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

6. Practicalities - lab computers
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

7. Science
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

8. Experiments
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?

9. Experiments 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

10. Stroop (1935)
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
Actually, he had published on the effect a few years earlier, but in German and few people have read that paper.

11. Stroop (1935)
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

12. A variant of the task
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

13. Try it for yourself RED BLUE GREEN YELLOW
Read out the colour of the letters in the words below
RED
BLUE
GREEN
YELLOW

14. And again… BLUE YELLOW RED GREEN
Read out the colour of the letters in the words below
BLUE
YELLOW
RED
GREEN

15. Conducting the task on a computer
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!

16. Experimental design 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

17. PsychoPy
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

18. PsychoPy basics 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)

19. Measure the Stroop effect
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)