The term ‘‘base rate’’ refers to the ‘‘prevalence of an event, such as a symptom, sign,
or disorder, within a given population’’ (McCaVrey, Palav, O’Bryant, & Labarge,
2003, pg. 1). Meehl and Rosen (1955) Wrst demonstrated the importance of base rates
in psychological testing nearly 50 years ago. Since that time there has been a wealth
of literature published regarding the importance of this information during clinical
decision making. One reason base rates are so important is because in clinical
practice, it is rarely the case that the probability of having certain conditions (e.g.,
depression, schizophrenia, PTSD, Alzheimer’s dementia, malingering) when present-
ing with certain symptoms is equal for all of the patients seen. The likelihood that a
person has a speciWc condition given certain symptoms Xuctuates with a variety of
factors such as age, gender, and race. The prevalence of dementia in individuals over
the age of 85 ranges from 20% to 50% (Heilman & Valenstein, 2003); stated another
way, one out of every Wve to every other person within the United States over the age
of 85 suVers from some form of dementia. The prevalence of dementia in individuals
under the age of 85 is diVerent; notably, lower. Therefore, clinicians must use base
rate information to determine the conWdence they can have in diagnosing a particular
patient with a speciWc condition given the symptoms they report. That is, without
appropriate base rate estimates, clinicians cannot estimate their diagnostic accuracy.
As Gouvier et al (2002) pointed out, ‘‘The eVect of base rates operates to skew
diagnostic accuracy in favor of predictions to the more prevalent category and to
reduce accuracy to the less prevalent category’’ (pg. 377).
Download here: Symptome Base Rates in the Gerneral Population
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