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Experimental Design: Introduction. Martin, Chapter 1. Psychology as a science.

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psychology as a science
Psychology as a science
  • “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
Methods for Investigating Hypotheses(Establishing Cause and Effect Relationships)
  • Correlational research
  • Quasi-experimental research
  • Experimental research

Quantitative vs. Qualitative research

kinds of variables
Kinds of Variables
  • Independent Variables
    • what you manipulate
  • Dependent Variables
    • what you measure
  • Control Variables
    • what you hold constant
  • Random Variables
    • what you allow to vary randomly
  • Confounding variable
    • correlated with independent variable
correlation
Correlation
  • Take two measures from a sample; calculate correlation coefficient
  • Possible questions:
    • is there a relationship between smoking and lung cancer?
    • is there a relationship between anxiety and test-taking performance?
  • Correlation does NOT imply causation
experiments
Experiments
  • Systematically vary variables of interest
    • e.g., giving different drugs
  • Critical concepts
    • Variable must be manipulated by experimenter
    • Random assignment of participants to conditions
    • Avoid confounding variables
quasi experimental
Quasi-experimental
  • Separate participants based on some characteristic, e.g.:
    • Gender, occupation, verbal ability (VSAT)
  • Possible questions
    • Do people with high verbal ability learn new languages faster?
  • 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
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
Randomization
  • Random Sampling
    • Choose participants randomly from the entire population
    • Allows generalization to population
  • Random Assignment
    • Methods for achieving random assignment
      • Flip coins; random numbers on arrival
      • Assign conditions in blocks