Experimental Design: Introduction Martin, Chapter 1

Psychology as a science n “Psychology is the Science of Mental Life, both of its phenomena and their conditions. The phenomena are such things as we call feelings, desires, cognitions, reasonings, decisions and the like; and, superficially considered, their variety and complexity is such as to leave a chaotic impression on the observer.” William James (1890)

Methods for Investigating Hypotheses (Establishing Cause and Effect Relationships) n Correlational research n Quasi-experimental research n Experimental research Quantitative vs. Qualitative research

Kinds of Variables n Independent Variables – what you manipulate n Dependent Variables – what you measure n Control Variables – what you hold constant n Random Variables – what you allow to vary randomly n Confounding variable – correlated with independent variable

Correlation n Take two measures from a sample; calculate correlation coefficient n Possible questions: – is there a relationship between smoking and lung cancer? – is there a relationship between anxiety and test- taking performance? n Correlation does NOT imply causation

Example of correlation

Experiments n Systematically vary variables of interest – e.g., giving different drugs n Critical concepts – Variable must be manipulated by experimenter – Random assignment of participants to conditions – Avoid confounding variables

Example of experiment

Quasi-experimental n Separate participants based on some characteristic, e.g.: – Gender, occupation, verbal ability (VSAT) n Possible questions – Do people with high verbal ability learn new languages faster? n Accepted, but effects may be due to another factor – e.g., high-verbal people went to better high-school

How to select a kind of study 1) representativeness – does this seem like the real thing? 2) control – can you manipulate something? 3) cost & effort – is it worth it to do it the painful way 4) availability – are the best kinds participants available?

Randomization n Random Sampling – Choose participants randomly from the entire population – Allows generalization to population n Random Assignment – Methods for achieving random assignment n Flip coins; random numbers on arrival n Assign conditions in blocks

Demo: Can you pick a random number? 1234