Lecture 9 Psyc 300A

Correlational Studies Why we do them –Ethical limits on experiments and participant variables –Often generalize well to other situations and people (external validity) Two major purposes –Finding relationships Remember, can’t get at causality. Why? –Making predictions Criterion variable Predictor variable These studies require design considerations like –Operational definitions –Appropriate sampling

Correlation coefficients Comparing pairs of scores Scatterplots Correlation coefficent –Ranges from -1.0 to +1.0 –Direction of relationship –Strength of relationship Pearson product-moment correlation is for interval and ratio data Potential problems –Nonlinearity –Restriction of range

Scatterplot

Return to Experiments Logic of the experiment (IV and DV) Review of –Extraneous variable –Confounding variable –Difference between the two

Properties of Studies Internal Validity –Extent to which observed relationships in a study (scores) reflect relationships between hypothetical variables. External Validity –Extent to which the results of a study can generalize to other people and settings outside the study

Designing Good IVs Strong vs weak manipulations –Difference between levels of IVs must be big enough to see effects on DV Examples of strong manipulations Manipulation checks: –A measurement, separate from the DV, to see if IV had its intended effect

Designing Good DVs Sensitive measure –A measure that is able to detect subtle differences in behavior Restriction of range –Floor effects: when task/test is too difficult, most scores will approximate the lowest possible score –Ceiling effects: when task/test is too easy, most scores will approximate the highest possible score