Clinical Psychology

Of course, all these single-subject designs, by
definition, deal with one person. Can we generalize
what has been shown true of one person to an
entire population? As with case study methods,
the external validity of the results or attempts to
generalize about them can be problematic. But as
long as we are interested in one specific person or
are seeking evidence that will encourage us subse-
quently to initiate a full-blown traditional experi-
mental study, the method has great merit.

Mixed Designs

Experimental and correlational techniques are
sometimes combined into a mixed design. Here,
participants who can be divided into specific popu-
lations (e.g., schizophrenic vs. normal) are assigned
as groups to each experimental condition. In this
way, variables such as psychosis or normality are
not manipulated or induced by the investigator.
Instead, they are correlated with the experimental
condition.

Davison et al. (2004) provide an excellent

hypothetical example of how mixed designs work.

Suppose we decide to investigate the efficacy of

three forms of therapy (the experimental manipula-

tion). We do this by identifying psychiatric patients

who can be divided into two groups on the basis of

the severity of their illness (the classificatory vari-

able). Does effectiveness of treatment vary with

severity of illness? The results of this hypothetical

research are shown in Figure 4-6. Figure 4-6(b)

presents data obtained when the patients were

divided into two groups according to the severity

of their illness. Figure 4-6(a) shows how confused

we could become had patients not been divided

into two groups. When all patients are combined,

we find that treatment 3 produced the largest

improvement, and we are mistakenly led to believe

that this treatment is the best. But when we analyze

the data according to severity of illness, treatment 3

is not the preferred one for either group of patients.

Instead, as Figure 4-6(b) shows, for patients with

less severe problems, treatment 1 is desirable,

10

9 8 7 6 5 4 3 2 1 0

12 3

Amount of improvement

Treatments

(a)

10

9 8 7 6 5 4 3 2 1 0

123

Amount of improvement

Treatments

(b)

Group with

severe illness

Combined

group

Group with less

severe illness

F I G U R E 4-6 Effects of three treatments on patients whose problems vary in degree of severity

SOURCE: From Abnormal Psychology, 7th ed., by G. C. Davidson and J. M. Neale, p. 117. Copyright © 1998 by John Wiley & Sons, Inc.
Reprinted by permission.

RESEARCH METHODS IN CLINICAL PSYCHOLOGY 117