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Research methods in clinical psychology: An introduction for students and practitioners Chris Barker, Nancy Pistrang, and Robert Elliott CHAPTER 8 Foundations of design
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Design: overview Types of designs Validity analysis Descriptive and correlational designs –Correlation and causation Quasi-experimental designs Randomised experimental designs –Control and comparison groups
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Classification of designs Non-experimental designs –Descriptive –Correlational Experimental designs –Non-randomised (quasi-experimental) –Randomised
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Causal relationships Central principle: Correlation does not mean causation Suppose that A and B are correlated: what might be the causal relationship between them? ctd./
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Causal relationships 2 Third variable: AB C Simple causation: A B BA ctd./
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Causal relationships 3: mediating and moderating variables Mediating variable: ADB Moderating variable: A B E (Baron & Kenny, 1986)
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Validity analysis Statistical conclusion Internal Construct External (from Cook and Campbell, 1979)
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Statistical conclusion validity Is the study sensitive enough? –statistical power –design quality Do the variables covary? If so, how strongly?
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Internal validity If two variables do covary, is there a causal relationship between them?
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Construct validity Do the outcome variables (and also the experimental intervention) represent the underlying constructs that they are supposed to?
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External validity Do the results of the study generalise (e.g., across settings or participants)?
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Some non-randomised designs (Cook & Campbell, 1979) (Notation: X = intervention; O = observation) One-group posttest-only design X0 One group pretest-posttest design O 1 XO 2
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Some non-randomised designs (cont.) Nonequivalent groups posttest-only design NRXO NRO Nonequivalent groups pretest-posttest design NRO X O NRO(Y) O
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Some threats to internal validity Endogenous change Maturational trends Reactivity of measurement Secular drift Interfering events Regression to the mean Selection effects
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Some threats to the construct validity of the intervention Confounding variables –“non-specific factors” Expectancy effects –“placebo effects” Hawthorne effect
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Randomised designs Essential feature: randomised assignment to experimental groups (conditions) This controls for the internal validity threat of selection effects, which is a problem with quasi-experimental (non-randomised) designs
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RCTs RCT is a common abbreviation for either: Randomised Controlled Trial or Randomised Clinical Trial Efficacy (as opposed to effectiveness) research
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Example “Randomised groups pretest-posttest design” RO X O RO(Y) O (see Cook & Campbell, 1979)
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Randomised designs: terminology The dependent variable(s) are the outcome measures The independent variable(s) are the experimental conditions The statistical method is analysis of variance (ANOVA)
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Factors in randomised designs Independent variables are arranged as factors each has two or more levels multifactorial designs within-group (repeated measure) factors between-group factors (experimental conditions) blocking factors (participant differences)
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Some types of control group No-treatment controls –treatment better than nothing Wait-list controls –control for expectation of benefit “Placebo” controls –credible but inert treatment –“double- and triple-blind” Comparative treatment groups –Alternative treatment –“Treatment as usual” Dismantling studies
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Practical limitations of randomisation Non-equivalence of groups Attrition –“intent to treat” analysis Leakage Non-cooperation of staff Costly and time consuming Can’t study negative events Ethical issues
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Ethical issues in RCTs No treatment/placebo controls Wait-list controls Random assignment v. choice Specified treatments v. clinical judgements Decision on patient inclusion Referrals at termination
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Some good experimental design features (1) Patient homogeneity Randomised assignment Groups similar after randomisation Specific interventions –manualisation Appropriate control groups Groups treated equivalently except for intervention ctd./
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Good design features (2) Low attrition Patients, clinicians and raters blind Follow-up after termination (e.g., 6 months, one year) Independent replication
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