Prof Phil Rees
Prof Paul Boyle
Dr. Corinne Camilleri-Ferrante
1st October 1999 - 30th September 2003
ESRC CASE Studentship
A fundamental question underlying many health studies is whether the risk of a certain disease in a small area is greater than that which is expected in comparison with the national or regional experience (Arnold 1999). Risk becomes apparent through an excess of cases over the number expected with health inequalities being identified by looking at the spread of rates of an event across small areas within a larger geographical area. For disease analysis and mapping it is necessary to calculate reliable numerators of disease incidence and denominator ‘populations at risk’ for the geographical areas of interest. Unfortunately, population migration leads to various methodological problems which hinder the derivation of reliable incidence rates.
Firstly, outside census years, the production of cross-sectional population estimates to use as denominator populations is difficult because migration patterns are complex. Whilst births and deaths can be estimated for small areas quite reliably, a lack of regularly updated migration flow data for small areas makes the migration component in population estimates notoriously unreliable, particularly those produced some time after the decennial census.
Secondly, most epidemiological studies ignore the fact that people move. Any attempts to relate disease incidence with the environment (using area-based deprivation scores or point- or line-based environmental hazards) make the implicit assumption that the cases have lived in an area long enough to be influenced by local environmental factors. The reality is that a person’s health status is the cumulative result of any cultural and environmental factors associated with their residential history. Unfortunately, due to the unavailability of detailed, individual-level migration histories, it is rarely known whether a person with a particular health condition has lived at the same address all their life, or whether they moved into an area shortly before the condition was diagnosed.
Thirdly, migration is a selective process that is influenced by many factors including an individual’s age and health status. This selectivity potentially affects both the numerator event data if an area receives or loses unusually high numbers of unhealthy migrants, as well as the denominator if an area receives or loses certain demographic subsets so that, for example, the age structure of the population at risk is changed.
These difficulties were identified some time ago (Rees and Wilson 1977; Prothero 1977) and simple mapping exercises have demonstrated differences that the inclusion or exclusion of migrants can make to the understanding of disease distributions (Kliewer 1992). However, whilst much attention has been paid to the accuracy of the numbers of disease cases in small areas, there has been relatively little consideration of possible errors in the population at risk figures used to derive disease rates as well as the potential contribution of selective migration on health inequalities between places and over time.
The aims of this project are: firstly, to provide a series of small area population estimates with an explicit migration element that can be used as denominator ‘populations at risk’ in health and medical studies; and secondly, to determine the differing morbidity and mortality experiences of migrants in comparison with non-migrants and whether healthy and unhealthy migrants tend to move to different types of locations. Finally, to use this information to evaluate the effect of population migration on health statistics: the extent to which migration may be causing denominator uncertainties whilst biasing the apparent health of an area and thereby contributing to health inequalities. Eastern Region will be used as the study area and where applicable, health variations in coronary heart disease will be considered.
The research is timely and topical. The Labour Government which came to power in 1997 acknowledges that poverty and deprivation are key determinants of health and are explicitly focusing on the alleviation of inequalities in health (Jowell 1997). Noting that health inequalities vary significantly between one place and another and from one social group to another, the Acheson Report (1998), establishes national targets for public health priorities including a reduction in the death rate from coronary heart disease for people under the age of 75 (Alberti 2000). Moreover, ongoing investigations into rare conditions such as CJD (Dillner 1996) and the publication of the Baker Report into Harold Shipman’s malpractices (Carvel and Morris 2001) demonstrate a need for reliable and versatile disease and death rates.
The project which commenced in October 1999 involves collaboration between the School of Geography at the University of Leeds and the Anglia Clinical Audit and Effectiveness Team (ACET) at the Institute of Public Health, University of Cambridge. Supervision is by Prof. Phil Rees at the University of Leeds, Prof. Paul Boyle at the University of St. Andrews and Dr. Corinne Camilleri-Ferrante at ACET.
References
Acheson D (1998) Our Healthier Nation. Reducing Health Inequalities: an Action Report (the ‘Acheson Report’), The Stationery Office, London
Alberti G (2000) National Service Framework for Coronary Heart Disease, Department of Health, London
Arnold R (1999) ‘Small Area Health Statistics Unit procedures for estimating populations in small areas’ in Population counts in small areas: implications for studies of environment and health, Studies on Medical and population Subjects no. 62 edited by Richard Arnold, Paul Elliott, Jon Wakefield and Mike Quinn, Proceedings of a meeting held on 11th December 1997, Small Area Health Statistics Unit, Imperial College of Science, Technology and Medicine, The Stationery Office, London, pp. 10-24
Carvel J and Morris S (2001) ‘Death in the afternoon - the chilling routine of a killer’, The Guardian, 06/01/01
Dillner L (1996) ‘BSE linked to new variant CJD in humans’, British Medical Journal, 312, p. 795, online eBMJ www.bmj.com/cgi/content/full/312/7034/795, accessed 10/02/01
Jowell, T (1997) Minister for Public Health, speech at the Conference of the Faculty of Public Health Medicine, Liverpool, 26th June 1997
Kliewer E V (1992) ‘Influence of migrants on regional variations of stomach and colon cancer mortality in the western United States’ International Journal of Epidemiology, Vol. 21, No. 3, pp. 442-49
Prothero R (1977) ‘Disease and mobility: a neglected factor in epidemiology’, International Journal of Epidemiology, 6, pp. 259-67
Rees P H and Wilson A (1977) Spatial population analysis, Edward Arnold, London