Introduction to experimental design

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Introduction to experimental design
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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

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Within participants

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

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Overview

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

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Mixed design

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

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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

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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

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Why distinguish

• We need to choose the appropriate statistical
  test
• Between unrelated or uncorrelated t test
• Within - related or correlated t-test

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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

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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

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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
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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

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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

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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.

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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

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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
16ml
10
6
2
pre-test post-test
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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

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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 dont come back !!

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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 lt .05

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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 lt .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.

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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
• Womens 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.

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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.

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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.