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

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

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England & Wales Sweden

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England & Wales Sweden

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