First year practicals

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

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

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

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

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 • http://webct.nottingham.ac.uk

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

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

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?

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

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

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

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

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

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

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!

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

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 www.psychopy.org

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)

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)

First year practicals