ch8 between-subjects desing
TRANSCRIPT
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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 causalrelationships?
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 timeof day)
Confounding variables extraneous variables that changesystematically 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 becomeconfounding variables that effect the dependent variable?
Can you have more than one dependent variable?
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Questions
Are errors in research thought of as being a third-variable (such as environmental or participantchanges)? I guess I dont really understand thedifference 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 toethics?
Are we skipping chapter 7 because 7 was on thesyllabus but today we did chapter 8.
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Experimental Designs: Between-
subjects design
Chapter 8
Dusana RybarovaPsyc 290B
May 24 2006
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Outline:
1. Introduction Characteristics of between-subject design
2. Advantages and disadvantages ofbetween-subjects designs
3. Within and between treatments variability
4. Other threats to internal validity of
between-subjects designs5. Applications and statistical analyses of
between-subjects designs
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1. Introduction Characteristics of
between-subject design
There are two basic research designs associatedwith 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 alist of one-syllable words, and then the same set of individuals istested again using a list of two-syllable words
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1. Introduction Characteristics of
between-subject design
the defining characteristic of a between-subjectsdesign is that it compares separate groups ofindividuals
another feature of a between-subjects design isthat it allows only one score per participant (everyscore represents a separate, unique participant)
because each score represents a separate
participant, a between subjects design is oftencalled an independent-measures design
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1. Introduction Characteristics of
between-subject design a between-subjects experimental design
requires a separate, independent group ofindividuals for each treatment conditioncompared
individuals are assigned to groups using aprocedure that attempts to create equivalentgroups
the general goal of between-subjects experiment
is to determine whether differences existbetween two or more treatment conditions (e.g.a researcher may want to compare two teachingmethods (two treatments) to determine whetherone is more effective than the other)
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2. Advantages and disadvantages of
between-subjects designs
Advantages
each individual score is independent of the
other scores
participants score is not influenced by such
factors as:
practice or experience gained in other treatments
fatigue or boredom from participating in a series oftreatments
contrast effects that result from comparing one
treatment to another (e.g. room temperature)
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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 confoundingvariables
individual differences can produce high variability in
the scores
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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
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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
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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 participantsto those with IQs between 100-110)
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3. Within and between treatments
variability
advantage
variability betweentreatments
it can be increased by
increasing differences
between conditions
(levels)
disadvantage
variability withintreatments
it is caused by
individual differences
should be minimized
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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 problemsassociated with high variability
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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 researchstudy before it is completed
differential attrition refers to differences in attrition ratesfrom one group to another and can threaten the internalvalidity of a between-subjects experiment (e.g.effectiveness of a dieting program)
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4. Other threats to internal validity of
between-subjects designs
diffusion or imitation of treatment
refers to the spread of the treatment effectsfrom the experimental group to the control
group (e.g. new depression therapy) compensatory equalization
occurs when an untreated group learns aboutthe treatment being received by another groupand demands the same or equal treatment (e.g.watching Batman in violent TV group)
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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 worksextra 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 lessproductive and less motivated because they resent theexpected superiority of the treated group
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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 determinewhether there is a significant difference between themeans
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 forstatistical analysis
adding extra groups to a research study tends to reducethe differences between groups