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Experimental Designs Between-subjects design

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					                        Questions
 Is Exam 2 going to be cumulative or will it just cover the
  second part of the information?
 Are cause-and-effect relationships the same as causal
  relationships?
 Can you give a clear example of the difference between
  confounding variables and extraneous variables?
   – Extraneous variables – any variables other than the studied
     dependent and independent variables in a study (e.g. random time
     of day)
   – Confounding variables – extraneous variables that change
     systematically with the studied variables (e.g. time of day
     systematically varied with a treatment)
 Do researchers need to address extraneous variables in
  their study, or only when the extraneous variables become
  confounding variables that effect the dependent variable?
 Can you have more than one dependent variable?
                    Questions
 Are errors in research thought of as being a third-
  variable (such as environmental or participant
  changes)? I guess I don’t really understand the
  difference between a third-variable, errors, and
  extraneous variables.
  – Third-variable is a confounding variable and a
    confounding variable is a kind of extraneous variable
 Can manipulation be deceptive… If so can the
  manipulation be a problem when it domes to
  ethics?
 Are we skipping chapter 7 because 7 was on the
  syllabus but today we did chapter 8.
Experimental Designs: Between-
       subjects design
           Chapter 8


          Dusana Rybarova
              Psyc 290B
             May 24 2006
                 Outline:
1. Introduction – Characteristics of between-
   subject design
2. Advantages and disadvantages of
   between-subjects designs
3. Within and between treatments variability
4. Other threats to internal validity of
   between-subjects designs
5. Applications and statistical analyses of
   between-subjects designs
 1. Introduction – Characteristics of
         between-subject design
 There are two basic research designs associated
  with the experimental research strategy
  – between-subjects design
      we obtain each of the different groups of scores from a separate
       group of participants
      e.g. one group of students is assigned to teaching method A
       and a separate group to method B
  – within-subjects design
      different groups of scores are all obtained from the same
       sample of participants
      e.g. one sample of individuals is given a memory test using a
       list of one-syllable words, and then the same set of individuals is
       tested again using a list of two-syllable words
  1. Introduction – Characteristics of
        between-subject design
 the defining characteristic of a between-subjects
  design is that it compares separate groups of
  individuals
 another feature of a between-subjects design is
  that it allows only one score per participant (every
  score represents a separate, unique participant)
 because each score represents a separate
  participant, a between subjects design is often
  called an independent-measures design
1. Introduction – Characteristics of
      between-subject design
 a between-subjects experimental design
  requires a separate, independent group of
  individuals for each treatment condition
  compared
 individuals are assigned to groups using a
  procedure that attempts to create equivalent
  groups
 the general goal of between-subjects experiment
  is to determine whether differences exist
  between two or more treatment conditions (e.g.
  a researcher may want to compare two teaching
  methods (two treatments) to determine whether
  one is more effective than the other)
2. Advantages and disadvantages of
       between-subjects designs
 Advantages
  – each individual score is independent of the
    other scores
  – participant’s score is not influenced by such
    factors as:
      practice or experience gained in other treatments
      fatigue or boredom from participating in a series of
       treatments
      contrast effects that result from comparing one
       treatment to another (e.g. room temperature)
2. Advantages and disadvantages of
       between-subjects designs
 Disadvantages
  – large number of participants (problem with
    special populations)
  – individual differences
     characteristics that differ from one participant to
      another are called individual differences
     individual differences can become confounding
      variables
     individual differences can produce high variability in
      the scores
2. Advantages and disadvantages of
       between-subjects designs
 Confounding variables in between subjects
  designs
  – individual differences
      participant characteristics differ from one group to another
      e.g. the participants in one group may be older, smarter,
       taller etc. than the participants in another group
  – environmental variables
      characteristics of the environment differ between groups
      e.g. one group may be tested in a large room and another
       group in a smaller room
2. Advantages and disadvantages of
       between-subjects designs
 Equivalent groups
  – in a between-subjects experimental design, the
    researcher does have control over the
    assignment of individuals to groups
  – the separate groups must be:
     created equally
     treated equally (except for the treatment conditions)
     composed of equivalent individuals
2. Advantages and disadvantages of
       between-subjects designs
 Limiting confounding by individual differences
  – random assignment (randomization)
      a random process is used to assign participants to groups
  – matching groups (matched assignment)
      involves assigning individuals to groups so that a specific
       variable is balanced or matched across the groups (e.g. IQ)
  – holding variables constant
      simply hold the variable constant (e.g. restrict the participants
       to those with IQs between 100-110)
  3. Within and between treatments
              variability

 advantage                   disadvantage
  – variability between        – variability within
    treatments                   treatments
  – it can be increased by     – it is caused by
    increasing differences       individual differences
    between conditions         – should be minimized
    (levels)
  3. Within and between treatments
              variability
 minimizing variability within treatments
  – standardize procedures and treatment setting
  – limit individual differences by holding a
    participant variable constant
  – random assignment and matching
  – sample size
      using a large sample can help minimize the problems
       associated with high variability
4. Other threats to internal validity of
       between-subjects designs
 assignment bias
   – groups of participants are different before the treatments
   – the group assignment process produces groups with
     noticeably different characteristics
 differential attrition
   – attrition refers to participant withdrawal from a research
     study before it is completed
   – differential attrition refers to differences in attrition rates
     from one group to another and can threaten the internal
     validity of a between-subjects experiment (e.g.
     effectiveness of a dieting program)
4. Other threats to internal validity of
       between-subjects designs
 diffusion or imitation of treatment
  – refers to the spread of the treatment effects
    from the experimental group to the control
    group (e.g. new depression therapy)
 compensatory equalization
  – occurs when an untreated group learns about
    the treatment being received by another group
    and demands the same or equal treatment (e.g.
    watching Batman in violent TV group)
4. Other threats to internal validity of
       between-subjects designs
 compensatory rivalry
  – occurs when an untreated group learns about the
    treatment received by another group and then works
    extra hard to show that they can perform just as well as
    the individuals receiving the special treatment
 resentful demoralization
  – opposite of compensatory rivalry
  – occurs when an untreated group learns about the
    treatment received by another group and is less
    productive and less motivated because they resent the
    expected superiority of the treated group
  5. Applications and statistical analyses of
            between-subjects designs
 comparing only two groups of participants
  – this design is referred to as the single-factor two-group
    design or simply two group design
  – an independent-measures t test is used to determine
    whether there is a significant difference between the
    means
 comparing means for more than two groups
  – e.g. single factor multiple group design may be used
    and analysis of variance (ANOVA) would be used for
    statistical analysis
  – adding extra groups to a research study tends to reduce
    the differences between groups