Prevalence Rates & Disorders (DP IB Psychology)
Revision Note
Written by: Claire Neeson
Reviewed by: Lucy Vinson
What are Prevalence rates?
Prevalence rates refer to how common a specific disorder is within a defined population
Prevalence for, say, major depressive disorder (MDD) would be measured as the proportion of people in the UK who were diagnosed with MDD from, say, January 2021 to January 2022
Prevalence rates are measured using an exact, mathematical calculation as follows:
The total number of cases of MDD in a specific population is identified
This number is then divided by the total number of people from that specific population
An example of a (hypothetical) prevalence rate might be: in a population of 50,000 people 4,500 have been diagnosed with MDD which means that the prevalence rate for MDD within that population is 0.09 (4,500 ÷ 50,000 = 0.09) or 9,000 cases of MDD per 100,000 people
Prevalence is a measure of an illness that enables statisticians to determine the likelihood of any person within that population being diagnosed with that illness e.g. it is a predictive tool
Prevalence rates of MDD
MDD has a high prevalence across the world: the World Health Organisation (WHO) has estimated that 3.8% of the world population suffers from MDD at any given time (5% for adults and 5.7% for those in the 60+ age group)
The above figures translate to a total of 280,000,000 people with MDD out of the total world population
The prevalence rate for MDD in the UK is 0.17 (this accounts for 1 in 6 adults with MDD across the population of the UK)
MDD is more prevalent in women both in the UK and globally (15% of women seek treatment for MDD compared to 9% of men)
There are cultural variations in MDD prevalence: 1% in the Czech Republic; 9% in Chile; 16.9% in the USA
Examiner Tips and Tricks
It is a good idea to memorise some stats to help you answer a question on prevalence rates. You don’t need to have a huge list of prevalence rates at your fingertips but it will add authority and context to your exam response if you can quote a couple of key figures relating to prevalence rates for MDD and/or phobias.
Prevalence rates of phobias
Phobias have a worldwide prevalence rate of 7.4% according to the WHO with more women than men suffering from phobias
According to the National Institute of Mental Health (USA) an estimated 9.1% of adults in the US suffer from having a specific phobia at any given time
The rate of phobia prevalence is higher for women (12.2%) than for men (5.8%)
The data also found that an estimated 12.5% of American adults will experience a specific phobia at some point in their lives
The most prevalent phobia in the UK is fear of heights (acrophobia) as of September 2023 according to government statistics with 75% of those surveyed expressing some degree of fear related to high places
Other highly prevalent phobias in the UK are fear of snakes (ophidiophobia) at 52%; fear of flying (aerophobia) at 24% and fear of spiders (arachnophobia) at 18%
Some research suggests that prevalence rates for anxiety disorders such as phobias are higher than for other anxiety disorders such as PTSD and OCD.
Which studies investigate prevalence rates of MDD and phobias?
Abdoli et al. (2022) - prevalence rates of MDD
Frederikson et al. (1996) - prevalence rates of phobias
Both Abdoli et al. and Frederikson et al. (1996) can be found as Two Key Studies of Prevalence Rates - just navigate the Etiology of Disorders section of this site.
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