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Table 1 Potential confounders, method and time of data collection, and level of analysis

From: Socio-demographic factors associated with pet ownership amongst adolescents from a UK birth cohort

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