How often we come across something is a way to define abnormalities. Unusual behaviour can be seen as normal, and unusual as abnormal, this is Statistical infrequency.
What was the iq statistical infrequency test
Average iq is 100, most (68%) score 85 to 115. 2% score below 70 and thats abnormal and can get diagnosed with a disability. 130 plus is also abnormal
Evaluation for usefulness real world application
Usef in therapies, clinical practice to assess how bad individuals symptoms are. A diagnosis of Intelectual disability requires an iq below 70.
Beck depression inventory is used for depression and a score of 304 is severely depressed. It's useful in diagnosis
Evaluation, unusual characteristics can be positive
Infeequent characteristics can be positive and negative. For every person below 70 another is above 130, being severly depressed isn't seen as abnormal. This isn't efficient as the primary basis for defining abnormalities