The prevalence of intellectual disability: A comparison of national census and register records.
Service registers give a truer head-count of people with ID than census data.
01Research in Context
What this study did
Roy and team compared two head counts of people with intellectual disability in Ireland. They pulled the 2016 national census and the National Intellectual Disability Database for the same year.
They asked: do both lists name the same people? They looked county by county to see where the numbers matched and where they drifted apart.
What they found
The census listed almost twice as many people with ID as the service register. Some counties showed a small gap; others showed a canyon.
In short, the census over-counts for planning services. The register gives the tighter list of who actually needs supports.
How this fits with other research
Whitehouse et al. (2014) did a door-to-door count of young learners in one French county and found a large share had mild ID. Their careful field survey landed between Roy’s two Irish numbers, showing that method matters.
Johnson et al. (2009) tracked US institutional census records for 31 years and watched the resident total drop by a large share. Like Roy, they showed administrative data can miss or over-count people when definitions shift.
Dell'Armo et al. (2024) warned that low-quality studies can make diagnostic overshadowing look bigger than it is. Roy’s paper is a clear example of high-quality admin data that future reviews can trust.
Why it matters
If you plan services, fund programs, or write grants, use the service register, not the census. The register keeps you from over-estimating caseloads and wasting scarce hours. When you read county-level stats, ask which list they came from before you stake budgets on them.
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02At a glance
03Original abstract
BACKGROUND: International prevalence rates for intellectual disability vary widely with estimates often based on samples. In Ireland people with an intellectual disability are identified in the national census. Moreover, a national register of people receiving or requiring intellectual disability services is maintained and updated annually. AIMS: The prevalence rates from the census were contrasted with those from the register along with variations in prevalence across the 26 counties of Ireland. METHODS: 2011 and 2016 Census of Population prevalence per 1000 for children (aged 5-19 years) and adults (20 years and over) stratified by the 26 countries were contrasted with similar prevalence in the national register. Publically available data from the 2011 census in Northern Ireland and Scotland were obtained. RESULTS: The Irish census identified nearly twice as many children and adults than were on the national register. Prevalence rates also varied across the 26 counties; more so on the register than the census. The Irish census had lower prevalence rates than Northern Ireland but higher than Scotland. CONCLUSIONS: Determining the prevalence of intellectual disability is challenging due to variations in terminology. A national register has advantages over reliance on census data for service planning.
Research in developmental disabilities, 2019 · doi:10.1016/j.ridd.2019.03.009