ch8 between-subjects desing

7/30/2019 Ch8 Between-Subjects Desing

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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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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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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


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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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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Confounding variables in between subjects

designs


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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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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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


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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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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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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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


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

ch8 between-subjects desing

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