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A rule indicating that certain combinations should not appear in experiments. For example, if conducting a study looking at price and brand of car, showing premium brands with low prices may be prohibited.

Broadly speaking, there are two quite different reasons for creating prohibitions:

  1. To ensure that experiments are not difficult for respondents to answer. For example, if an experiment included a Manipulation reflecting different descriptions of a product, to see which description was more appealing, it is generally advisable to prohibit one respondent from seeing both descriptions.
  2. To ensure that experiments are consistent with the "real world". For example, if for cost reasons a telecommunications company would never try and create a phone that was both very cheap and had lots of features, the company may not want the Experimental Design to have such combinations.

Ensuring that an experimental design addresses the first of these considerations will increase the validity of the experiment. Ensuring that the experimental design addresses the second of these prohibitions may reduce the validity of the experiment (because the resulting increase in the Standard Errors that results from the prohibition may offset any benefit obtained).