1. Research methods in clinical psychology: An introduction for students and practitioners Chris Barker, Nancy Pistrang, and Robert Elliott CHAPTER 4 Foundations of quantitative measurement

2. Operationalisation Construct Measure(s) Construct is “latent”: (it can never be directly observed) Complications: Theory dependence of measurement Reactivity of measurement

3. Examples of measures by source and approach

4. Quantitative methods: overview General background –positivism Psychometric theory –reliability –validity –generalisability –item response theory –utility

5. Advantages of quantitative measures Precision Theory of reliability and validity Established statistical methods for data analysis Facilitates comparison Fits in well with hypothetico-deductive approaches

6. Positivism (August Comte) Restrict scientific attention to observables Apply the methods of the physical sciences to the social sciences Science is objective and value free

7. Psychometric theory “How good is this measure?” Theory of measurement

8. Scales of measurement Nominal –unordered categories Ordinal –order property only Interval –ordinal plus equal intervals Ratio –interval plus actual zero

9. Type of measure Nomothetic –Compares across people, e.g., WAIS, BDI Idiographic –Measures within the person, e.g., Q-sort, personal questionnaire

10. Classical test theory Classical test theory measurement model: observed score = true score + error Underpins the concepts of reliability and validity

11. Reliability Reproducibility of the measurement –do you obtain the same results each time? Equivalent to the proportion of error in the measurement

12. Types of reliability Test-retest reliability Equivalent forms reliability Split-half reliability –no longer used Internal consistency –assumes parallel items –(Cronbach’s alpha) Inter-rater reliability –(Cohen’s kappa; intraclass correlation)

13. Validity Meaning of the measure “Whether the measure measures what it is supposed to measure.”

14. Validity types: 1 Content validity –consistency with definition of construct Face validity –does it look right? Criterion validity –compare to “gold standard” –concurrent validity (present) –predictive validity (future) –sensitivity and specificity

15. Validity types: 2 Construct validity Cronbach & Meehl (1955) –validity is theory based Campbell & Fiske (1959) –convergent validity –discriminant validity –method variance –multitrait-multimethod matrix

16. Suggested standards ReliabilityValidity Good.80.50 Acceptable.70.30 Marginal.60.20 Poor.50.10

17. Alternative approaches Item response theory –uses item characteristic curves –e.g., Rasch Scaling Generalisability theory –Cronbach –uses an ANOVA framework

18. Utility General user friendliness –e.g., length and layout Incremental utility –what does the measure add? Cost-effectiveness