Page 174 - [Uma_Sekaran]_Research_methods_for_business__a_sk(BookZZ.org)

158 EXPERIMENTAL DESIGNS

however, to the real world of work, where the employees and the nature of
the jobs would both be quite different. Thus, subject selection and its interac-
tion with the treatment would also pose a threat to external validity. These are
just some of the factors that restrict generalizability. Maximum external validity
can be obtained by ensuring that, as far as possible, the lab experimental con-
ditions are as close to and compatible with the real-world situation. It is in this
sense that field experiments have greater external validity than lab experi-
ments. That is, the effects of the treatment can be generalized to other settings
that are similar to the one where the field experiment was conducted. In Chap-
ter 11, we will discuss generalizability as a function of sampling design.

REVIEW OF FACTORS AFFECTING INTERNAL
AND EXTERNAL VALIDITY

In summary, at least seven contaminating factors exist that might affect the inter-
nal validity of experimental designs. These are the effects of history, maturation,
testing, instrumentation, selection, statistical regression, and mortality. It is, how-
ever, possible to reduce the biases by enhancing the level of sophistication of the
experimental design. Whereas some of the more sophisticated designs, discussed
below, would help increase the internal validity of the experimental results, they
could also become expensive and time consuming.
Threats to external validity can be combated by creating experimental condi-
tions that are as close as possible to the situations to which the results of the
experiment are to be generalized. For more extensive discussions on validity, see
Cook and Campbell (1979b).
The different types of experimental designs and the extent to which internal
validity is met in each are discussed next.

TYPES OF EXPERIMENTAL DESIGNS
AND INTERNAL VALIDITY

Let us consider some of the commonly used experimental designs and determine
the extent to which they guard against the seven factors that could contaminate
the internal validity of experimental results. The shorter the time span of the
experiments, the less the chances are of encountering history, maturation, and
mortality effects. Experiments lasting an hour or two do not usually meet with
many of these problems. It is only when experiments are spread over an
extended period of say, several months, that the possibility of encountering more
of the confounding factors increases.

Quasi-Experimental Designs
Some studies expose an experimental group to a treatment and measure its
effects. Such an experimental design is the weakest of all designs, and it does