Statistical Deviation

• Using numbers
• Normal and abnormal are described according to the number of times it is observed.
• Behaviours that are statistically rare should be seen as abnormal.
• More than two standard deviations is statistically rare/infrequent
• Used in almost all medical diagnosis
• Strength of statistical infrequency 


  Real world application: can be appropriate in defining abnormality- like intellectual disability disorder.
• Intellectual disability disorder: very small subset of the population (>2.2%) have IQ below 70 pts.
• Limitations of statistical infrequency

  • Where do we draw the line between normal and abnormal?- depends on how far behaviour deviates from the “norm”.
  • Some behaviour is rare but desirable.
  • Not all abnormal behaviours are infrequent- about 10% of people will be chronically depressed at some point in their lives.