General Types of Statistical Studies

• Designed experiment.
  • The analyst chooses and controls range of factors.
  • Nuisance factors would be equalized via the randomized process.
• Observational study.
  • Factors of interest can not be controlled.
  • It is at the mercy of nature.
• Retrospective study.
  • Historical data are used.
  • Advantages:
    • no cost in collecting the data.
  • Disadvantages:
    • validity of data is often in doubt.
    • there may be data missing.
    • there may be unknown errors in data.
    • there is no control on the range of the measured variables.