Table 1.

Potential confounders, method and time of data collection, and level of analysis

Variable	Method and time of data collection	Levels
Ownership of a Cat, Dog, Rabbit, Rodent, Bird, Fish, Tortoise/turtles and Horse ownership	Collected retrospectively at 11, 13, 15 and 18 years	No, yes
Gender	Medical records at birth	Male or female
Ethnicity of child	Carer questionnaire at 140 months (11 years)	White, mixed, Asian, black, other. Collapsed to ‘white’ and ‘other’
Number of people in household	Derived from mother’s questionnaire at 122 months (10 years)	3, 4, 5+
Presence of an older sibling	Derived from mother’s questionnaire (child based) at 18 months	No, yes
Maternal education	Mother’s questionnaire at 32 weeks gestation. Highest level indicated	CSE or no qualification (lowest), vocational, O level, A level, degree (highest)
Paternal education	Mother’s questionnaire at 32 weeks gestation. Highest level indicated	CSE or no qualification (lowest), vocational, O level, A level, degree (highest)
Maternal social class	Derived from mother’s questionnaire at 32 weeks gestation (occupation)	Professional (highest), Managerial and technical, Skilled: non-manual, Skilled: manual, Partly skilled, Unskilled (lowest)
Paternal social class	Derived from mother’s questionnaire at 32 weeks gestation (occupation)	Professional (highest), Managerial and technical, Skilled: non-manual, Skilled: manual, Partly skilled, Unskilled (lowest)
Maternal age at delivery	Clinical records	Continuous (years) OR < 21 years, 21–30 years, > 30 years
Mother had pets as a child	Mother’s questionnaire at 33 months	No, not at all; Yes, part of time; Yes, always
House type	Derived from mother’s questionnaire at 122 months (10 years)	Detached, semi-detached, end terrace, terraced, flat/room in someone else’s house/other