Research Design in Communicative Disorders

[Dr. Linton Hutchinson]

Research design is the plan and structure of an investigation for the
purposes of obtaining answers to research questions. The basic purposes
of research designs are to provide answers and to control variables of
interest. An appropriate research design allows the investigator to
obtain answers to questions of research and to control the expiremental,
extraneous, and error variances of the research problem being studied.

There are two major classes of research designs: group designs and
single subject designs. In a group design, one or more groups of study
volunteers are exposed to one or more levels of the maniputable variable
(independent variable). The average performances of each groups are
then averaged to determine the relationship between the independent and
dependent variable. Single subject designs study the individual
behavior of subjects. Single subject and group designs differ in that
single subject designs do not look at average performance. Instead, the
data of each study volunteer is evaluated individually.

In evaluating a research design, there are two important factors the
evaluator needs to consider. These factors relate to the validity of
the design. The two important factors are internal validity and
external validity. Internal validity is defined as the degree to which
the design answers the research questions and controls variance
appropriately to provide an uncontaminated description of the
relationshhip between the independent and dependent variables. External
validity is defined as the degree to which generalizations can be made
outside of the confines of the study.