Prevalence of intellectual disability: a comprehensive study based on national registers.

Pitetti et al. (2007) counted every person with intellectual disability in Finland. They linked four national registers: health, social, pension, and special education. The team checked if one register alone would miss people.

Using only one register caught just a large share of citizens. Merging all four doubled the count to a large share. In plain words, half of the real cases were invisible in any single list.

Freeman et al. (2015) copied the multi-register trick in Sweden and added a map. They showed the same rule: more lists equal higher counts, especially in the rural north.

Hermans et al. (2014) looked at the same Finnish adults after they were found. Four out of five had two or more extra health problems. Together the papers say: first find everyone, then plan care.

Bhaumik et al. (2008) found that a large share of UK adults with ID used psychiatric services. Their number makes sense only if the true population is first known; H et al. give that base.

If you write grants, sit on planning boards, or allocate staff, demand linked data sets. Single agency records will low-ball prevalence and leave families waiting. Start every needs assessment with the question: which registers can we merge today?