Research methods in clinical psychology: An introduction for students and practitioners Chris Barker

1
Research methods in clinical psychologyAn
introduction for students and practitionersChris
Barker, Nancy Pistrang, and Robert Elliott

• CHAPTER 8
• Foundations of design

2
Design overview

• Types of designs
• Validity analysis
• Descriptive and correlational designs
• Correlation and causation
• Quasi-experimental designs
• Randomised experimental designs
• Control and comparison groups

3
Classification of designs

• Non-experimental designs
• Descriptive
• Correlational
• Experimental designs
• Non-randomised (quasi-experimental)
• Randomised

4
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./
5
Causal relationships 2
Simple causation
B
A
B
A
A
B
Third variable
C
ctd./
6
Causal relationships 3mediating and moderating
variables
Mediating variable
A
D
B
Moderating variable
A
B
E
(Baron Kenny, 1986)
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Validity analysis

• Statistical conclusion
• Internal
• Construct
• External
• (from Cook and Campbell, 1979)

8
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?

10
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)?

12
Some non-randomised designs(Cook Campbell,
1979)

• (Notation
• X intervention
• O observation)
• One-group posttest-only design
• X 0
• One group pretest-posttest design
• O1 X O2

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Some non-randomised designs (cont.)

• Nonequivalent groups posttest-only design
• NR X O
• NR O
• Nonequivalent groups pretest-posttest design
• NR O X O
• NR O (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
• R O X O
• R O (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
• Cant 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