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Experimental research designs are used for the controlled testing of causal processes. The general procedure is one or more independent variables are manipulated to determine their effect on a dependent variable. These designs can be used where: 1) There is time priority in a causal relationship (cause precedes effect), 2) There is consistency in a causal relationship (a cause will always lead to the same effect), and 3) The magnitude of the correlation is great. The most common applications of these designs in marketing research and experimental economics are test markets and purchase labs. The techniques are commonly used in other social sciences including sociology and psychology.
One of the most important requirements of experimental research designs is the necessity of eliminating the effects of spurious, intervening, and antecedent variables. 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 variables (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] 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.
A true experimental design requires an artificial environment so as to control for all spurious, intervening, and antecedent variables. A purchase laboratory approaches this ideal. Participants are given money, script, or credit to purchase products in a simulated store. Researchers modify one variable at a time (for example; price, packaging, shelf location, size, or competitors’ offerings) and determine what effect that has on sales volume. Internet based purchase labs (called virtual purchase labs) are becoming more common.
Simplified versions of the purchase laboratory are often used for pragmatic reasons. An example of this would be to use tachistocopes for testing packaging and shelf location.
Quasi-experimental designs control some, but not all, of the extraneous factors. A test market is an example of this. A new product is typically introduced in a select number of cities. These cities must be representative of the overall national (or international) population. They should also be relatively unpolluted by outside influences (for example : media from other cities). The marketer has some control over the marketing mix variables, but almost no control over the broader business environment variables. Competitors could change their prices during the test. Government could change the level of taxes. New competing products could be introduced. An advertising campaign could be initiated by competitors. Any of these spurious variables could contaminate the test market.
Experimental research designs
In an attempt to control for extraneous factors, several experimental research designs have been developed, including:
- Classical pretest-post test - The total population of participants is randomly divided into two samples; the control sample, and the experimental sample. Only the experimental sample is exposed to the manipulated variable. The researcher compares the pretest results with the post test results for both samples. Any divergence between the two samples is assumed to be a result of the experiment.
- Solomon four group design - The population is randomly divided into four samples. Two of the groups are experimental samples. Two groups experience no experimental manipulation of variables. Two groups receive a pretest and a post test. Two groups receive only a post test. This is an improvement over the classical design because it controls for the effect of the pretest.
- Factorial design - this is similar to a classical design except additional samples are used. Each group is exposed to a different experimental manipulation.
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