company. Sales at the end of three months increase by $10,000.
A movie theater adds more parking spaces in its lot, and 300 more tickets are sold in a
Goodyear adds a line of ultra-high performance radials and sells 200,000 more of
them in six months.
As a before-after design, the causal statement is that “X caused O.” That is, the frequent
cleaner plan caused $10,000 in sales; the parking spaces caused 300 tickets to be sold, and
that addition of the radials caused Goodyear to sell 200,000 more of them. However, students
will generate a great number of other possible reasons why the sales were as described. Other
independent variables may come into play, for example an especially popular movie shown
in the month. Extraneous variables are involved as well such as competitors’ actions or
changes in customers.
However, the control group aspect of this design measures the effects of the other
independent variables and the extraneous variables. When the before-after amounts for the
control groups are subtracted from the before-after amounts of their associated experimental
groups, the true effect of the company’s changed independent variable is apparent.
9. The descriptions of internal and external validity in the chapter do not utilize terms used in
more comprehensive treatments. For internal validity, the terms are history, maturation,
instrumentation, pretest effect, equivalence, and mortality, while for external validity; they
are sample representativeness, artificiality, and generalizability. Some instructors may want
to introduce these terms to students when reviewing the two types of validity.
10. There are experimental designs that are more complex and which are not covered in the
chapter. Some Instructors may want to expand students’ knowledge by bringing these into the
11. Students can relate to a taste test as an example of a laboratory experiment, and they can be
questioned on why, for example, when Diet Pepsi does a blind taste test against Diet Coke,
findings show a slight preference for Diet Pepsi. However, Diet Coke sales are more than
those for Diet Pepsi. The artificiality of the blind taste test can be exploited. There are
extraneous and independent variables affecting brand loyalty, plus who does a blind taste test
of two colas to decide which one to buy?
12. Consider doing an abbreviated taste test in class. You can select some students to be the
administrators, and two or three to be the taste testers. One approach is to tell the
administrators to read up on taste tests and to design and implement it themselves. Another is
to bring the materials to class and select the administrators and tasters from class, but you
will need to orient the administrators on how to conduct the test.
Here are the instructions for three colas with six subjects. It is best to use Dixie cup-sized
cups as subjects must take several drinks. It is fun to use national brands like Coke, or Pepsi