Introduction to experimental design

1. Introduction to experimental design

2. The psychology experiment • Predict the causal effect of one thing on another • Keep everything constant other than the affecting thing • Vary the affecting thing systematically • Measure changes in the affected thing • Assess statistically whether or not the predicted effect has occurred

3. Within participants • same participants in each condition • controls for individual differences • introduces order effects and carry-over effects

4. Overview • Between participants • different participants in each condition • no order effects • Individual differences • need to match or randomise allocation of participants

5. Mixed design Mixed design • some conditions have different participants, some have the same

6. Example- between subjects design • Comparing the number of errors made entering into a computer spreadsheet for a sample of people listening to loud popular music with the number of errors made by a different control sample listening to white noise. • Two different people are compared

7. Examplewithin-subjects design • Studying the number of keyboard errors made by a group of 20 secretaries, comparing the number of errors when music is played to when music is not played. • Performance of one group of people is compared in two different circumstances

8. Why distinguish • We need to choose the appropriate statistical test: • Between – unrelated or uncorrelated t – test • Within - related or correlated t-test

9. Why laboratory research ? • Practicalities: equipment/apparatus to bulky, security, expensive • Experimental control: keeping all factors the same • E.g. light, temp, noise, arrangement of equipment • These are extraneous or environmental factors

10. True or randomised experiment • Experimental manipulation: manipulated variable = independent variable • e.g. Alcohol • Alcohol increases the number of mistakes • The level/amount of alcohol = IV • Amount given to each subject is constant for each condition • Condition one = 8ml and condition two = 16ml • Lower quantity of alcohol = control condition • Higher quantity of alcohol = experimental condition

11. Full population of interest Randomly assign into control and experimental groups Experimental group:exposed to independent variable: view violent film Control group:View nonviolent film

12. Checks on experimental manipulation • Experiment on memory and anger • Researcher says pre-scripted offensive comments to people in the experimental group and nice things to the control group • Possible problems: • View it as a joke, patronising • Resolve the issue by either: • get subjects to complete of questionnaire on their mood • after debriefing ask how they felt about the researchers questions • Pilot

13. Standardisation of procedures • Keeping things constant Alcohol and error experiment • Time of day • Body weight of participants • Time they ate • Researchers behaviour • Any others ? • Resolutions • Tape recorded instructions • Come into lab previous day

14. Randomisation • Who goes in the experimental or control group • What if the participant undergoes more than one condition • Toss of a coin • more than two • Throw of a dice • Write on cards, random number tables, computer number generation • Problems: runs of the same condition or number of participants in either condition is different • Randomisation ensures that there is no systematic bias in the selection process of participants, although chance factors may lead to differences between the conditions.

15. Matching • Ensuring equal numbers • Matched block or block randomisation • First Ss of a pair is randomly assigned to control condition using the specified procedure, while other pair is assigned to remaining condition • We need to ensure that participants in the control and experimental condition are similar • Matching on gender, age weight

16. Pre-test and post test sensitisation effects Number Of errors • Without a pre-test there is only a measure of people Performance after drinking • But, look at the pre-test – maybe due to randomisation people who generally made more mistakes were in the 8ml group 8ml 14 10 16ml 6 2 pre-test post-test

17. Cont… • Having a pre-test helps us to determine whether randomisation worked • It allows us to see whether or not there has been a change in performance between the pre- and post test • Disadvantage • Alert the Ss to the purpose of the experiment • Solutions • Increase the length of intervals between the pre and post test • We could test participants again after the post test

18. Within-subjects design • Fatigue or boredom – number of mistakes maybe more in the second than in the first condition • Practice effect – Ss become better at task • Carryover, asymmetrical transfer – the effect of an earlier condition affects the subsequent condition. Solution increase time between conditions, but the problem is sometimes they just don’t come back !!

19. statistical significance • The key to determining if a treatment had an effect is to measure the statistical significance. • Statistical significance shows that the relationship between the variables is probably not due to mere chance and that a real relationship most likely exists between the two variables. Statistical significance is often represented like this: p < .05

20. Cont…. • A p-value of less than .05 indicates that the possibility that the results are due merely to chance is less than 5%. Occasionally, smaller p-values are seen such as p < .01. There are a number of different means of measuring statistical significance. The type of statistical test used depends largely upon the type of research design that was used.

21. Androgyny Androgyny Today we accept a lot more diversity (e.g. Hayley Cropper off Corrie) and see gender as a continuum (i.e. scale) rather than two categories. So men are free to show their “feminine side” and women are free to show their “masculine traits”. For example, • Beckman wears a skirt • Earrings for men • Women’s boxing • Girl Power So it has become a lot more difficult to say what us typically “male” or “female”, and people who are biologically one sex often possess qualities (and the behaviour) appropriate to the opposite sex.

22. Androgyny Refers to the recognition that individuals possess qualities (or traits) which are characteristic of both masculinity and femininity (Bem, 1974) Davison (2000) - women that those who had androgynous characteristics scored highly in terms of their well-being, than women that were not androgynous. Gana (2001) found that highly androgynous husbands had a happier home life and participated more in the household tasks and in the bringing up of the children than did husbands with rigid traditional gender views.

23. Questionnaire Take 10 minutes to complete this questionnaire and score it. Do not identify yourselves on the questionnaire Lets do your first psychological experiment !!!! We will use these results for our seminar session next week, and create a discussion section ourselves during the seminar session. I will provide you with the introduction and methods sections.