Issues in Design--Overview

Experimental versus Correlational designs

Experimental

Manipulation of (at least) one variable -- Independent Variable
Effect on (at least) one other variable -- Dependent Variable

Correlational

Observation of the relationship between two variables
Inability to determine causality

Quasi-Experimental -- Field studies

direct relevance
difficult to have appropriate controls

Standard Experimental Designs and the control of extraneous influences

Within subjects

controls for subject variability
confounds condition with practice and order effects
- order or practice
  - fatigue
  - learning
  - differential transfer
- counterbalancing as a control
  - all orders
  - some orders
  - two orders
- counterbalancing controls main effects of order, not interactions

Between subjects

eliminates order and practice effects
confounds subject characteristics with condition
- subject characteristics as a confound
  - ability
  - motivation
- randomization as a control
  - matching
  - randomization
  - block randomization

Mixed designs -- Within and Between

multiple conditions within subject
counterbalancing across subjects

Part of a course in Research Methods in Psychology (Psychology 205) at Northwestern University, developed by William Revelle.
Back to the course syllabus.
Revised April 16, 2002