International comparisons of social gradients in child health: methodological issues A case study of infant mortality Philippa Bird and Kate Pickett, University of YorkISCI conference, July 2011

Outline Background to PhD project The social gradient in infant mortality in Sweden and England and Wales Comparability of infant mortality How is infant mortality measured? Are there measurement differences? How does this affect our interpretation of comparisons of the social gradient in infant mortality? Conclusions

Why study the social gradient in child health and development in relation to income inequality?

There is a social gradient in child health and development Marmot (2010)

Child health and wellbeing are worse in more unequal countries

Does everyone do better in more equal countries? There is some evidence that The social gradient in health is steeper in more unequal countries The benefits of income equality are widespread across the social gradient. Although the least well-off benefit the most from greater equality, there are also benefits among the most well-off. But Evidence remains limited, in particular for children There are counter-examples The extent to which these differences are an artefact of differences in measurement is unclear.

How comparable are indicators of child health and development? How comparable are indicators of child socioeconomic position? Does everyone do better in more equal countries? Literature review Secondary data analysis

Literature review Studies that compare the social gradient in more and less equal countries 14 studies, of which: 5 focus on child health/development 2 analyse role of income inequality

Infant mortality is lower across all social classes in Sweden, which is more equal than England Source: Leon et al 1992

How comparable is infant mortality in Sweden and England and Wales? a) How is infant mortality measured? b) Are there international differences in the way infant mortality is measured? c) Does this affect my interpretation of the social gradient in infant mortality in Sweden and England and Wales? Does it affect comparisons of the overall level of infant mortality? Does it affect comparisons of the social gradient in infant mortality?

a) How is infant mortality measured? Number of deaths during the first year of life per 1,000 live births. (WHO)

b) Are there international differences in the way infant mortality is measured? When an infant dies there are three possible classifications: Live birth followed by an infant death Stillbirth No classification Affected by: i) The way infant deaths are classified in the registration system ii) The coverage of the registration system IMR

i) Differences in classification Infant deaths As number of infant deaths is small, small discrepancies have a large impact (Liu and Moon, 1992) Live births Small direct effect by altering the denominator. Larger indirect effect on infant death recording Stillbirths Indirect effect on infant death and live birth recording … particularly among preterm infants or in first 24 hours of life

Official differences in classification Requirements for reporting a live birth in Europe and the US, 2004 (MacDorman and Mathew, 2009)

Informal differences in classification Why? Ease burden on parents (Draper and Field, 2007) Cultural differences in management of pre-term infants (Draper and Field, 2007, MacDorman and Mathew, 2009) Different estimation of gestational age

Quantifying the impact of differences in registration Data on live births Are there fewer very preterm live births than expected? Data on infant deaths Disaggregated by gestational age – are there fewer infant deaths among very preterm infants than expected? Disaggregated by age - are there fewer infant deaths recorded in the first 24 hours than expected? Data on stillbirths Are there more then expected?

Quantifying the impact of differences in registration Proportion of live births that weigh less than 500g varied 50-fold variation between Sweden, Israeli Jews, US blacks (Kramer et al., 2002) Infant mortality rates among very pre-term infants (22-23 weeks) varied widely, from 515/1000 live births in Sweden to 1000/1000 in Scotland and Northern Ireland in 2004 (MacDorman and Mathew, 2009)

Source: Gourbin and Masyuy- Stroobant, 1995

Standardised perinatal mortality rates Source: Graafmans et al., 2001

ii) Differences in registration coverage Include or exclude non-nationals? UK: all infant births and deaths Sweden: may exclude births and deaths to people living in the country temporarily (Ludvigsson et al. 2009)

c) Does this affect my interpretation of the social gradient in infant mortality in Sweden and England and Wales? Does it affect comparisons of the overall level of infant mortality? Does it affect comparisons of the social gradient in infant mortality?

Neonatal mortality Postneonatal mortality Source: Leon et al., 1992

Conclusions For the PhD methods Take account of differences in measurement for: Interpretation of previous studies comparing levels and social gradients in child health/development Choice of indicators for secondary data analysis and interpretation of findings Even for seemingly objective indicators For international comparisons of infant mortality Interpret with caution! Postneonatal mortality may be more comparable