78 CHAPTER 3
Problems concerning reliability in diagnosis can occur when:- the criteria for disorders are unclear, and thus require the clinician to use consid-
erable judgment about whether symptoms meet the criteria; or - there is signifi cant overlap among disorders, which can then make it diffi cult to
distinguish among them.
However, just because clinicians agree on a diagnosis doesn’t mean that the
diagnosis is correct! For example, in the past, there was considerable agreement
about the role of the devil in producing mental disorders, but we now know that
this isn’t a valid explanation. Science is not a popularity contest; what the ma-
jority of observers believe at any particular point in time is not necessarily cor-
rect. Thus, another requirement for any classifi cation system is that it needs to be
valid—the categories must characterize what they are supposed to be classifying.
Each disorder should have a unique set of criteria that are necessary for the diag-
nosis to be made.
The reliability and validity of classifi cation systems are important in part be-
cause such systems are often used to study the etiology of a psychological disorder,
itsprognosis (the likely course and outcome of the disorder), and whether particu-
lar treatments will be effective. In order to use a classifi cation system in this way,
however, the prevalence of each disorder—the number of people who have the dis-
order in a given period of time—must be large enough that researchers are likely to
encounter people with the disorder. (A related term is incidence, which refers to the
total number of new cases of a disorder that are identifi ed in a given time period.)
In addition, to be valid, the criteria must not be too restrictive or too broad.
If the criteria for diagnosing a particular disorder are so restrictive that the dis-
order’s prevalence is very low, not much will be discovered about its course or
the most effective treatments. On the other hand, if the criteria for a disorder
are so broad that the disorder is very common, the criteria may refl ect a set of
different (but perhaps related) problems or even include aspects of normal—not
abnormal—psychological functioning; in either case, the classifi cation is not of
much use (Kutchins & Kirk, 1997).
In sum, a classifi cation system should be as reliable and valid as possible in
order to be useful for patients, clinicians, and researchers. Next, we’ll examine the
most commonly used classifi cation system for psychological disorders—the system
in the Diagnostic and Statistical Manual of Mental Disorders (DSM).The Diagnostic and Statistical Manual
of Mental Disorders
Suppose that child welfare offi cials had spoken with Rex and Rose Mary Walls
and required that the parents be evaluated by mental health professionals. How
would a mental health clinician go about determining whether or not either
of them had a disorder? What classifi cation system would the clinician proba-
bly use? The classifi cation system that most clinicians use in the United States
is found in the Diagnostic and Statistical Manual of Mental Disorders, which is
currently in its fourth edition. This guide, published by the American Psychiatric
Association (2000), describes the characteristics of many psychological disorders
and identifi es criteria—the kinds, number, and duration of relevant symptoms—
for diagnosing each disorder. This classifi cation system is categorical—someone
either has a disorder or does not.
A different classifi cation system, described in the International Classifi cation of
Diseases (ICD), is used in some other parts of the world. The World Health Or-
ganization (WHO) develops the ICD, which is currently in its tenth edition. The
primary purpose of the ICD is to provide a framework for collecting health sta-
tistics worldwide. Unlike the DSM, however, the ICD includes many diseases andValid
Classifi cation systems (or measures) that
actually characterize what they are supposed
to characterize.
Prognosis
The likely course and outcome of a disorder.
Prevalence
The number of people who have a disorder in
a given period of time.