Prevalence varies by age because of cohort differences from birth and early infancy Larger or smaller cohorts move progressively through the age

groups. In the United Kingdom the largest cohort, born in the early 1960s, was aged 30+ in 1998.

5. Prevalence varies by age and time because survival has increased at all ages. In developed communities there are currently more adults over 45 than children under 15 and substantial numbers of elderly severely intellectually impaired people.

6. Prevalence and mortality data available permit rough planning estimates of age-specific prevalence ratios in developed countries. These data assume a stable population over the last half century and no unusual factors affecting large aetiological groups. The youngest and oldest groups are only informed guesses (Table 1..).

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per 1044

IE

1/1 r»D

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ihum

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2 Zti

10-14

4.0

1.75

2.10

11-14

4 S

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1

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4.20

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15. 19

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15-54

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2 55

2.»

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1.0

2.10

2.30

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TH-

V.rv f.w

Table 1 Estimates of prevalence of severe intellectual impairment (IQ < 50) per 1000 for a stable population

7. There are usually more males than females at all ages. The ratio varies between 1:1 and 2:1, but with no clear pattern. It probably depends upon the local spectrum of causes of brain damage and disorder, with differential incidence and mortality.

8. Recent studies have tended to show a social class gradient consistent with the known social distribution of morbidity and mortality. Previous differences in inceptions were masked by early mortality.

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