12 Intellectual disability12.2 Epidemiology
12.2.1 Interpreting intellectual disability occurrence
2 min read
Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
What is being counted
Before you compare figures, check what kind of count you are looking at.
Population prevalence counts everyone in a defined population who meets the case definition at a set time. This includes people who are not known to specialist services.
Administrative prevalence counts the people recognised on a register or by a service. It reflects access to services, how cases are recorded and who is eligible, as well as how common the condition really is.
The case definition needs impairment of both intellectual and adaptive functioning, starting during development. A low test score is not enough on its own. Neither is an education support category.
Population estimates are usually higher than service counts. Some people, especially those with milder disability, are never identified or stay outside specialist services.
Onset and recognition
Measure | What it means |
|---|---|
Point prevalence | Existing cases divided by the population at that time |
Incidence proportion | New cases during a set period divided by the people at risk at the start |
Incidence rate | New cases divided by the total time at risk built up over the period |
Newly recorded diagnoses | The first time a case is recognised in the dataset, which may be long after the disability began in development |
Onset and recognition are different things.
Recognition can rise at school age, when the demands of education bring difficulties to light. This does not mean the intellectual disability began at that age.
A diagnosis made in adulthood may pick up a lifelong developmental condition. New cognitive decline is different: assess it for an acquired disease.
Prevalence can change because of survival, migration, diagnostic practice and ascertainment (how cases are found). So a rising register count does not, by itself, show that incidence is rising.
Illustrative register calculation
These figures are invented. They show how recording measures work and are not real population estimates. Assume a fixed population, with no deaths, migration or removals from the register during the year.
Item | Number |
|---|---|
Residents | 10,000 |
Residents recorded with intellectual disability at baseline | 100 |
Further residents first recorded during the year | 20 |
Baseline recorded prevalence = 100 / 10,000 = 0.01, or 1%.
Newly recorded diagnoses = 20 / 10,000 = 0.002, or 2 per 1,000 residents during the year.
End-of-year recorded prevalence = 120 / 10,000 = 0.012, or 1.2%.
These data cannot give the incidence of developmental onset, because the dates when each person's disability began are unknown.
Comparing figures
Before you read a difference between two figures as a real change in occurrence, make sure both use the same age range, case definition, population boundary and method of ascertainment.