AIM To determine whether those most easily reviewed in a population prevalence study differ from those followed up only with difficulty.
METHODS All babies born before 32 weeks of gestation in the North of England in 1983, 1990, and 1991 were traced, and all the survivors assessed at two years by one of two independent clinicians.
RESULTS 818 of the 1138 live born babies survived to discharge. There was some non-significant, excess disability in the 5% of long term survivors who were difficult to trace because of social mobility, but eight times as much severe disability in the 1% (9/796) in care and in the 5% (38/796) whose parents initially failed to keep a series of home or hospital appointments for interview, and five times as much emergent disability in the 2.7% (22/818) who died after discharge but before their second birthday. Had the babies who were seen without difficulty been considered representative of all the babies surviving to discharge, the reported disability rate would have been two thirds what it really was (6.9% instead of 11.0%).
CONCLUSIONS Population prevalence studies that ignore those who seem reluctant to cooperate risk serious ascertainment bias.
Children seen for review without difficulty in any study of community prevalence are seldom representative of those who are not assessed
Failure to review those who prove difficult to trace introduces less bias than failure to review those who prove difficult to review once traced
Studies where it is not possible to see some children for assessment might usefully include a calculation of what the total prevalence would be if there was a fivefold difference in the proportion with the condition in question among the children who were not seen
Parental support for any such study can nearly always be obtained if an effort is made to make the family feel, from the outset, that they are partners in a collaborative exercise of genuine worth
- prevalence studies
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