4.1.4 Definitions of Abnormality: Statistical Infrequency

Implies that a disorder is abnormal if its frequency is more than two standard deviations away from the mean incidence rates represented on a normal distribution bell curve.

• Some characteristics are regarded as abnormal even though they are quite frequent
• Some infrequent behaviours are desirable e.g., photographic memory Can be used to diagnose intellectual disability disorder. All assessments of patients with mental disorders include some sort of measurement of its severity compared to the norm. Seen as valid due to its use in everyday situations.

A downside of this approach is that extremes can be viewed as either positive or negative. For instance, an IQ score of 130 or higher is statistically abnormal, but high intelligence is not considered undesirable. Just because someone exhibits certain behaviours does not necessarily mean they need treatment. Therefore, there are significant limitations to using statistical infrequency alone as a basis for diagnosis.