Types of research design – experimentsChapter 8 in Babbie & Mouton (2001)
Introduction to all research designsAll research designs have specific
objectives they strive forHave different strengths and
limitationsHave validity considerations
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Validity considerations
When we say that a knowledge claim (or proposition) is valid, we make a JUDGEMENT about the extent to which relevant evidence supports that claim to be true
Is the interpretation of the evidence given the only possible one, or are there other plausible ones?
"Plausible rival hypotheses" = potential alternative explanations/claims
e.g. New York City's "zero tolerance" crime fighting strategy in the 1980s and 1990s - the reverse of the "broken windows" effect
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The logic of causal social research in the controlled experiment
Explanatory rather than descriptive Different from correlational research - one
variable is manipulated (IV) and the effect of that manipulation observed on a second variable (DV)
If … then …. E.g.
"Animals respond aggressively to crowding" (causal) "People with premarital sexual experience have more
stable marriages" (noncausal)
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Three pairs of components:
Independent and dependent variables
Pre-testing and post-testingExperimental and control groups
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Components
VariablesDependent (DV)Independent (IV)
Pre-testing and post-testingO X O
Experimental and control groupsTo off-set the effects of the experiment
itself; to detect effects of the experiment itself
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The generic experimental design:
R O1 X O2
R O3 O4
The IV is an active variable; it is manipulated
The participants who receive one level of the IV are equivalent in all ways to those who receive other levels of the IV
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Sampling
1. Selecting subjects to participate in the researchCareful sampling to ensure that results
can be generalized from sample to population
The relationship found might only exist in the sample; need to ensure that it exists in the population
Probability sampling techniques
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Sampling
2. How the sample is divided into two or more groups is importantto make the groups similar when they
start offrandomization - equal chancematching - similar to quota sampling
proceduresmatch the groups in terms of the most
relevant variables; e.g. age, sex, and race
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Variations on the standard experimental design
One-shot case study
X O
No real comparison
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A famous one-group posttest-only design
Milgram's study on obedienceObedience to authorityThe willingness of subjects to follow E's orders
to give painful electrical shocks to another subject
A real, important issue here: how could "ordinary" citizens, like many Germans during the Nazi period, do these incredibly cruel and brutal things?
If a person is under allegiance to a legitimate authority, under what conditions will the person defy the authority if s/he is asked to carry out actions clearly incompatible with basic moral standards?SUMBER: web.uct.ac.za/.../Types%20of%20research%20d...
One-group pre-test post-test design
O1 X O2
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Example
We want to find out whether a family literacy programme enhances the cognitive development of preschool-age children.
Find 20 families with a 4-year old child, enrol the family in a high-quality family literacy programme
Administer a pretest to the 20 children - they score a mean of say 50 on the cognitive test
The family participates in the programme for twelve months
Administer a post-test to the 20 children; now they score 75 on the test - a gain of 25
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Two claims/conclusions:
1 The children gained 25 points on average in terms of their cognitive performance
2 the family literacy programme caused the gain in scores
VALIDITY - rival explanations
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Static-group comparison
X O O
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Evaluating research (experiments)
We know the structure of researchWe understand designsWe know the requirements of "good"
researchThen we can evaluate a studyIs it good? Can we believe its
conclusions?Back to plausible rival hypotheses
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Validity in designs
If the design is not valid, then the conclusions drawn are not supported; it is like not doing research at all
Validity of designs come in two parts:Internal validity
can the design sustain the conclusions?External validity
can the conclusions be generalized to the population?
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Internal validity
Each design is only capable of supporting certain types of conclusions e.g. only experiments can support conclusions about
causality Says nothing about if the results can be
applied to the real world (generalization) Generally, the more controlled the
situation, the higher the internal validity The conclusions drawn from experimental
results may not accurately reflect hat has gone on in the experiment itself
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Sources of internal invalidity
These sources often discussed as part of experiments, but can be applied to all designs (e.g. see reactivity)
HistoryHistorical events may occur that will be
confounded with the IVEspecially in field research (compare the
control in a laboratory, e.g. nonsense syllables in memory studies
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Maturation
Changes over time can be caused by a natural learning process
People naturally grow older, tired, bored, over time
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Testing (reactivity)
People realize they are being studied, and respond the way they think is appropriate
The very act of studying something may change it
In qualitative research, the "on stage" effects
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The Hawthorne studies
Improved performance because of the researcher's presence - people became aware that they were in an experiment, or that they were given special treatment
Especially for people who lack social contacts, e.g. residents of nursing homes, chronic mental patients
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Placebo effect
When a person expects a treatment or experience to change her/him, the person changes, even when the "treatment" is know to be inert or ineffective
Medical research"The bedside manner", or the power
of suggestion
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Experimenter expectancy
Pygmalion effect - self-fulfilling prophecies of e.g. teachers' expectancies about student achievement
Experimenters may prejudge their results - experimenter bias
Double blind experiments: Both the researcher and the research
participant are "blind" to the purpose of the study.
They don't know what treatment the participant is getting
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Instrumentation
Instruments with low reliability lead to inaccurate findings/missing phenomena
e.g. human observers become more skilled over time (from pretest to posttest) and so report more accurate scores at later time points
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Statistical regression to the mean
Studying extreme scores can lead to inflated differences, which would not occur in moderate scorers
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Selection biases
Selection subjects for the study, and assigning them to E-group and C-group
Look out for studies using volunteers
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Attrition
Sometimes called experimental (or subject) mortality
If subjects drop out, it creates a bias to those who did not e.g. comparing the effectiveness of family therapy with
discussion groups for treatment of drug addiction addicts with the worst prognosis more likely to drop out
of the discussion group will make it look like family therapy does less well than
discussion groups, because the "worst cases" were still in the family therapy group
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Diffusion or imitation of treatments
When subject can communicate to each other, pass on some information about the treatment (IV)
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Compensation
In real life, people may feel sorry for C-group who does not get "the treatment" - try to give them something extrae.g. compare usual day care for street
children with an enhanced day treatment condition
service providers may very well complain about inequity, and provide some enhanced service to the children receiving usual careSUMBER: web.uct.ac.za/.../Types%20of%20research%20d...
Compensatory rivalryC-group may "work harder" to
compete better with the E-group
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Demoralization
Opposite to compensatory rivalryMay feel deprived, and give up
e.g. giving unemployed high school dropouts a second chance at completing matric via a special education programme
if we assign some of them to a control group, who receive "no treatment", they may very well become profoundly demoralized
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External validity
Can the findings of the study be generalized?
Do they speak only of our sample, or of a wider group?
To what populations, settings, treatment variables (IV's), and measurement variables can the finding be generalized?
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External validity Mainly questions about three aspects:
Research participants Independent variables, or manipulations Dependent variables, or outcomes
Says nothing about the truth of the result that we are generalizing
External validity only has meaning once the internal validity of a study has been established
Internal validity is the basic minimum without which an experiment is uninterpretableSUMBER: web.uct.ac.za/.../Types%20of%20research%20d...
External validity Our interest in answering research questions is
rarely restricted to the specific situation studied - our interest is in the variables, not the specific details of a piece of research
But studies differ in many ways, even if they study the same variables: operational definitions of the variables subject population studied procedural details observers settings
Generally bigger samples with valid measures lead to better external validity
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Sources of external invalidity
Subject selection - Selecting a sample which does not represent the population well, will prevent generalization
Interaction between the testing situation and the experimental stimulus
When people have been sensitized to the issues by the pre-test
Respond differently to the questionnaires the second time (post-test)
Operationalization SUMBER: web.uct.ac.za/.../Types%20of%20research%20d...
Operationalization We take a variable with wide scope
and operationalize it in a narrow fashion
Will we find the same results with a different operationalization of the same variable?
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Field experiments
"natural" - e.g. disaster researchStatic-group comparison type Non-equivalent experimental and
control groups
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Strengths and weaknesses
StrengthsControlManipulating the IVSorting out extraneous variables
Weaknesses Articifiality - a generalization problemExpenseLimited range of questions
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