Statistical infrequency

• Statistical infrequency

  Defines abnormality as behaviour that deviates from the mean average, thus the less often the behaviour occurs (statistically) the more likely it is to be abnormal
• The majority of people are normal and a minority are abnormal according to the statistical infrequency definition
• Normal distribution


  Concept used to decide which behaviour is statistically normal and which is statistically abnormal
• The 'big five' personality characteristics (e.g. extraversion, neuroticism) are normally distributed, and mental illness is thought to be extremes of these traits
• Approximately 68% of people fall within one standard deviation (SD) from the mean, 95% fall within 2 SD, and 99.73% fall within 3 SD
• Standard deviation


  A measure of dispersion which shows how far spread out the data is from the mean
• Scores more than two SD away from the mean denote an abnormality
• The DASS (depression, anxiety and stress scale) uses a questionnaire to measure depression, anxiety and stress, with a very high score considered 'abnormal' and in need of treatment
• Strengths of the statistical infrequency definition

  • Gives a quantitative, objective measure with a clear cut-off point for abnormality
  • Reduces stigma as no judgements are made by the clinician
• Limitations of the statistical infrequency definition

  • Infrequent behaviour can be positive (e.g. high IQ)
  • Mental illness can be frequent (e.g. depression in 10% of population)
• Statistical infrequency should be looked at alongside other definitions (FFA, distress) to identify behaviour that needs treatment