1.1.5 - Principles of Experimental Design

The following principles of experimental design have to be followed to enable a researcher to conclude that differences in the results of an experiment, not reasonably attributable to chance, are likely caused by the treatments.

Need to control for effects due to factors other than the ones of primary interest.
Subjects should be randomly divided into groups to avoid unintentional selection bias in the groups.
A sufficient number of subjects should be used to ensure that randomization creates groups that resemble each other closely and to increase the chances of detecting differences among the treatments when such differences actually exist.

The benefits to randomization are:

  1. If a random assignment of treatment is done then significant results can be concluded as causal or cause and effect conclusions. That is, that the treatment caused the result. This treatment can be referred to as the explanatory variable and the result as the response variable.
  2. If random selection is done where the subjects are randomly selected from some population, then the results can be extended to that population. The random assignment is required for an experiment. When both random assignment and selection are part of the study then we have a completely randomized experiment. Without random assignment (i.e.an observational study) then the treatment can only be referred to as being related to the outcome.