Causal Research Design: Experimentation
Concept of Causality
A statement such as "X causes Y " will have the following meaning to an ordinary person and to a scientist. ____________________________________________________ Ordinary Meaning Scientific Meaning ____________________________________________________ X is the only cause of Y. X is only one of a number of possible causes of Y.
X must always lead to Y (X is a deterministic cause of Y).
It is possible to prove that X is a cause of Y.
The occurrence of X makes the occurrence of Y more probable (X is a probabilistic cause of Y).
We can never prove that X is a cause of Y. At best, we can infer that X is a cause of Y.
Conditions for Causality
Concomitant variation is the extent to which a cause, X, and an effect, Y, occur together or vary together in the way predicted by the hypothesis under consideration. The time order of occurrence condition states that the causing event must occur either before or simultaneously with the effect; it cannot occur afterwards. The absence of other possible causal factors means that the factor or variable being investigated should be the only possible causal explanation.
Definitions and Concepts
Independent variables are variables or alternatives that are manipulated and whose effects are measured and compared, e.g., price levels. Test units are individuals, organizations, or other entities whose response to the independent variables or treatments is being examined, e.g., consumers or stores. Dependent variables are the variables which measure the effect of the independent variables on the test units, e.g., sales, profits, and market shares. Extraneous variables are all variables other than the independent variables that affect the response of the test units, e.g., store size, store location, and competitive effort.
An experimental design is a set of procedures specifying
the test units and how these units are to be divided into homogeneous subsamples, what independent variables or treatments are to be manipulated, what dependent variables are to be measured, and how the extraneous variables are to be controlled.
Validity in Experimentation
Internal validity refers to whether the manipulation of the independent variables or treatments actually caused the observed effects on the dependent variables. Control of extraneous variables is a necessary condition for establishing internal validity. External validity refers to whether the cause-and-effect relationships found in the experiment can be generalized. To what populations, settings, times, independent variables and dependent variables can the results be projected?
History refers to specific events that are external to the experiment but occur at the same time as the experiment. Maturation (MA) refers to changes in the test units themselves that occur with the passage of time. Testing effects are caused by the process of experimentation. Typically, these are the effects on the experiment of taking a measure on the dependent variable before and after the presentation of the treatment. The main testing effect (MT) occurs when a prior observation affects a latter observation.
In the interactive testing effect (IT), a prior measurement affects the test unit's response to the independent variable. Instrumentation (I) refers to changes in the measuring instrument, in the observers or in the scores themselves. Statistical regression effects (SR) occur when test units with extreme scores move closer to the average score during the course of the experiment. Selection bias (SB) refers to the improper assignment of test units to treatment conditions. Mortality (MO) refers to the loss of test units while the experiment is in progress.
Controlling Extraneous Variables
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