1.13.A—Identify elements of a welldesigned experiment

Identify elements of a welldesigned experiment.

• A well-designed experiment should include the following:
  • i. Comparisons of at least two treatment groups, one of which could be a control group
  • ii. Random assignment of treatments to experimental units
  • iii. Replication
  • iv. Direct control of potential extraneous sources of variation in the response
• A control group is a collection of experimental units that are created for comparison. A control group may be given a treatment different from the treatment of interest to determine if the treatment of interest has an effect (e.g., a treatment with an inactive substance, a placebo, may be given).
• The placebo effect is the difference between the average response to a placebo and the average response to no treatment.
• In a single-blind, also called single-masked, experiment, participants do not know which treatment they are receiving, but members of the research team who interact with them know which treatment each participant is receiving, or vice versa.
• In a double-blind, also called double-masked, experiment, neither the participants nor the members of the research team who interact with them know which treatment each participant is receiving.
• An extraneous source of variation, also referred to as an extraneous variable, is a variable that is known (or believed) to affect the response but is not an explanatory variable being studied.
• The purpose of random assignment is to create treatment groups that are as similar as possible with respect to extraneous sources of variation. If random assignment is successful, the respective distributions of each extraneous variable will be approximately the same for all the treatment groups.
• A confounding variable in an experiment is a variable that is related to the explanatory variable in such a way that it is difficult to determine which variable, explanatory or confounding, is influencing the change in the response variable. However, in a well-designed experiment, the potential for confounding variables is reduced.
• Replication within an experiment means more than one experimental unit is assigned to each treatment.
• Direct control in an experiment means keeping the settings of certain potential extraneous sources of variation in the response variable the same from experimental unit to experimental unit.