Experimental design random assignment of participants to groups. The effects on the dependent variable of manipulating one or more independent variables are investigated. Non-experimental design - actions or events carefully measured and catalogued, but independent variables cannot be directly manipulated i. e. , quasiexperimental, correlational, survey, and single-subject or small-n research. Scientific understanding entails two distinct but related processes: description and explanation. Operational definition indicates how a concept is coded, measured, or quantified: erkman: the facial action coding system provides the operational definition of various facial expressions of emotion. The coding system defines specific combinations of facial muscle movements that are universally and discretely generated when certain emotions are elicited. An important aspect of a scientific description and explanation is establishing predictive relationships between two events or occurrences. Explanation can include both prediction as well as establishing cause and effect. It is impossible in human studies to control for all extraneous, cofounding or third-variables that could account for the observed effect.