Editing Design of experiments (section)

==Statistical control==
It is best that a process be in reasonable statistical control prior to conducting designed experiments.  When this is not possible, proper blocking, replication, and randomization allow for the careful conduct of designed experiments.<ref>Bisgaard, S (2008) "Must a Process be in Statistical Control before Conducting Designed Experiments?", ''Quality Engineering'', ASQ, 20 (2), pp 143–176</ref>
To control for nuisance variables, researchers institute '''control checks''' as additional measures.  Investigators should ensure that uncontrolled influences (e.g., source credibility perception) do not skew the findings of the study.  A [[manipulation checks|manipulation check]] is one example of a control check.  Manipulation checks allow investigators to isolate the chief variables to strengthen support that these variables are operating as planned.

One of the most important requirements of experimental research designs is the necessity of eliminating the effects of [[spurious relationship|spurious]], intervening, and [[antecedent variable]]s. In the most basic model, cause (X) leads to effect (Y). But there could be a third variable (Z) that influences (Y), and X might not be the true cause at all. Z is said to be a spurious variable and must be controlled for. The same is true for [[intervening variable]]s (a variable in between the supposed cause (X) and the effect (Y)), and anteceding variables (a variable prior to the supposed cause (X) that is the true cause). When a third variable is involved and has not been controlled for, the relation is said to be a [[zero order (statistics)|zero order]] relationship. In most practical applications of experimental research designs there are several causes (X1, X2, X3). In most designs, only one of these causes is manipulated at a time.