Table 1.

Potential risk factors and confounders, method and time of data collection, and factor level of analysis.

Variable	Method and time of data collection	Level
Cat	Derived from number of cats in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Dog	Derived from number of dogs in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Rabbit	Derived from number of rabbits in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Rodent	Derived from number of rodents in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Bird	Derived from number of birds in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Fish	Derived from number of fish in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Tortoises/turtles	Derived from number of tortoises/turtles in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Other pet	Derived from number of other pets in mother’s questionnaire at gestation, 8, 21, 33, 47, 85, 97 and 122 months	No, yes
Gender	Medical records	Male or female
Ethnicity	Carer questionnaire (child-based) at140 months	White, mixed, asian, black, other. Collapsed to ‘white’ and ‘other’.
Number of people in household	Derived from mother’s questionnaire at gestation, 8, 21, 33, 47,85, 97, 122 months.	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 gestation, 8, 21, 33,85, 122 months	Detached, semi-detached, end terrace, terraced, flat/room in someone else’s house/other (note for gestation no end terrace category)