Research methods in clinical psychology: An introduction for students and practitioners Chris Barker, Nancy Pistrang, and Robert Elliott CHAPTER 8 Foundations of design

Design: overview Types of designs Validity analysis Descriptive and correlational designs –Correlation and causation Quasi-experimental designs Randomised experimental designs –Control and comparison groups

Classification of designs Non-experimental designs –Descriptive –Correlational Experimental designs –Non-randomised (quasi-experimental) –Randomised

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./

Causal relationships 2 Third variable: AB C Simple causation: A B BA ctd./

Causal relationships 3: mediating and moderating variables Mediating variable: ADB Moderating variable: A B E (Baron & Kenny, 1986)

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

Statistical conclusion validity Is the study sensitive enough? –statistical power –design quality Do the variables covary? If so, how strongly?

Internal validity If two variables do covary, is there a causal relationship between them?

Construct validity Do the outcome variables (and also the experimental intervention) represent the underlying constructs that they are supposed to?

External validity Do the results of the study generalise (e.g., across settings or participants)?

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

Some non-randomised designs (cont.) Nonequivalent groups posttest-only design NRXO NRO Nonequivalent groups pretest-posttest design NRO X O NRO(Y) O

Some threats to internal validity Endogenous change Maturational trends Reactivity of measurement Secular drift Interfering events Regression to the mean Selection effects

Some threats to the construct validity of the intervention Confounding variables –“non-specific factors” Expectancy effects –“placebo effects” Hawthorne effect

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

RCTs RCT is a common abbreviation for either: Randomised Controlled Trial or Randomised Clinical Trial Efficacy (as opposed to effectiveness) research

Example “Randomised groups pretest-posttest design” RO X O RO(Y) O (see Cook & Campbell, 1979)

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)

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)

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

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

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

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./

Good design features (2) Low attrition Patients, clinicians and raters blind Follow-up after termination (e.g., 6 months, one year) Independent replication