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Experiments, Treatments and Experimental Units

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A comparative experiment involves:

  • Selecting a sample from a population
  • Dividing a sample into groups or blocks
  • Applying a different treatment to each group
  • Testing the research hypotheses using statistical methods

An experimental unit is a physical entity or subject exposed the the treatment independently of other units.

Experimental error is the variation among identically and independently treated experimental units.

The notion of an experiment implies a degree of control, such as selection of the sample and assignment of subjects to the treatment groups. An example would be a drug study, when different dosage and drug (or placebo) is administered to different pens of laboratory animals selected for the study.

Comparative observational studies are conducted when it is either impractical or impossible to conduct an experiment. An example would be the observation of the street traffic passing through traffic-light controlled intersections, where the traffic lights are working according to different rules.