Bias from non-participation: a study of low-level lead exposure in children.

Epidemiological studies on neurobehavioural effects in childhood are highly sensitive to selection bias. A study on the significance of low-level lead exposure was performed among school children in the first grade in Aarhus in 1982-83. Deciduous teeth were collected and analysed for the circumpulpal dentin lead concentration. On the basis of the lead levels, a case and a control group were identified. Information on socioeconomical status and past medical history was obtained for all children. A "common sense" stratification of the reason for non-participation, showed bias in particular due to non-responders, while "refusers" caused little, if any bias. A non-participation of 7% significantly distorted the main result of the study in the direction of the null-hypothesis. Current studies of low-level lead exposure vary greatly with regard to non-participation and the extent of selection bias may contribute to understand the apparent diversity of findings of studies in this field.

The effects of lead exposure on urban children: the Institute of Child Health/Southampton Study.

A study of the associations between level of tooth lead, behaviour, intelligence and a variety of other psychological skills was carried out in the child population aged six to seven years in three London boroughs. Tooth lead was estimated from the chemical analysis of shed teeth donated by children. 2663 (62.4 per cent) of the eligible children donated teeth. A study of the total population was carried out to see if those who donated teeth were representative of that population. There were small but consistent and statistically significant differences--tooth-givers being of slightly higher intelligence and showing fewer behaviour problems. 403 children, selected on the basis of their tooth-lead levels and social class, were studied more intensively. They were classified into six pre-arranged groups--high, medium and low tooth-lead levels, with each lead group divided into two social groups, manual and non-manual. The parents of these children were intensively interviewed in their homes regarding parental interest and attitudes to education, family characteristics and relationships, the early history of the child and the child's physical environment. The intelligence of the mother was measured. The child was then studied in school using tests of intelligence, educational attainment and other cognitive tasks. Teachers and parents completed standardised behaviour questionnaires. The results showed that intelligence and other psychological measures were strongly related to social factors, especially social grouping. Lead level was linked to a variety of factors in the home, especially the level of cleanliness, and to a lesser extent, maternal smoking. There was no significant link between lead level and behaviour, though when rated by teachers, but not by parents, there were small and reasonably consistent non-significant tendencies for high-lead children to show more difficult behaviour. Before social factors were controlled for, there were significant differences between the lead groups in measures of intelligence and two other psychological tests, the children in the high-lead groups performing worse. Once a number of social factors had been taken into account, the differences between the three lead groups (high, medium and low) became small and statistically nonsignificant, although they remained in the same direction.(ABSTRACT TRUNCATED AT 400 WORDS)