Queen Mary & Westfield College, Department of Geography, Mile End Road, London, E1 4NS, UK. E-mail: p.congdon{at}qmul.ac.uk
The authors of the Stockholm study1 in this issue of the International Journal of Epidemiology are continuing the recent trend in epidemiology to recognize the impact that ecological and other contexts may have on health outcomes beyond those due to individual risk factors and behaviours.2 This trend has accompanied improved technical methods (e.g. multi-level and random effects models) that facilitate such an approach. The authors recognize the need for the qualifications material deprivation, as measured by the Townsend index... and social fragmentation as measured by the Congdon index. The operationalization of theoretical constructs can never be perfect and indices used in ecological studies are recognized to be imperfect measures of constructs.3 A related problem is conceptual overlap in constructs and even in dependent and independent variables. An interesting example at the individual rather than ecological level is the recognition in studies4 linking illness to sense of coherence (SOC) that items in an SOC scale may also partly be measures of the outcome (e.g. psychological well-being) that it is intended to predict.
The Stjärne et al. study is particularly concerned with the validity of indices for two ecological concepts, deprivation and social fragmentation. My own attempt to summarize fragmentation derived from work on suicide variations in London in the 1980s and early 1990s in which it was clear there were influences apart from deprivation.5 In London the highest suicide rates for this period were in mostly affluent central London areas (Kensington & Chelsea, Camden and Westminster) with a distinctive household and demographic structure (many non-family households, high population mobility, much private renting in bedsitters, etc). More recent work has applied this type of index to psychiatric morbidity in general.6 In recent years (since 1998) the spatial pattern of suicide in London has changed, especially for males, but that is another story.
The Stockholm study application of this fragmentation index to physical rather than mental ill health follows similar work by Davey Smith et al.7 The authors caution against uncritical application of the fragmentation and deprivation indices and I would support them in this. My own view is that the two concepts may be positively correlated but are distinct, so I would differ on the suggestion that they partly measure the same underlying phenomenon. The fragmentation index is likely also to be correlated to the familism factor which is well established in studies of urban factorial ecology.8 Further well established is the separation of familism from the deprivation-affluence factor. One may say that an index of social integration (in positive sense) or of social fragmentation (negative sense) is seeking to measure social integration and social support resting on non-institutional ties (with partners, families) and on institutional and community ties. Social integration is more usually associated with stable family lifestyles, relatively low population turnover, and possibly with religious and/or ethnic community structures. Its counterpart would be characterized by social isolation, weak kin and community ties, and a non-family household bias (e.g. many single person households). As some have argued,9,10 a social integration/support construct should be distinguished from a social disorder scale based on adverse neighbourhood perceptions, environmental decay, fear of crime, and neighbourhood risk or stress. While social disorder may well be positively correlated with deprivation, social fragmentation may occur in affluent areas (e.g. central London) as well as deprived areas.
High correlations between indices may be explained by considering what each constituent of the index measures. In London household overcrowding (a constituent of the Townsend index) may in part be related to deprivation. However, overcrowding is also high in affluent central London and reflects a chosen lifestyle by mobile young adults early in professional careers. Similarly car ownership, another element in the Townsend index, may reflect urbanity-rurality (and urban centrality versus suburban location) rather than deprivation.11 My own work12 showed that replacing overcrowding by a social class measure and omitting car ownership (resulting in a measure closer to the Carstairs index) reduced the correlation between deprivation and fragmentation in London (1991 Census data) from 0.75 to 0.27. The Stockholm authors note that cohabitation may be increasingly making marital status outmoded, so one could substitute two parent families with one or more dependent children (negatively weighted if one were deriving a fragmentation index) for proportion single, widowed, and divorced.
To illustrate recent patterns, deprivation and fragmentation scores were derived for 354 English local authorities from the 2001 UK Census. Fragmentation scores are defined as in the Stockholm study, but the deprivation scores12 combine routine occupations (lower skill manual), social housing, and unemployment. The correlation between the two sets of scores is 0.08. Quintiles of both scores were obtained and standardized mortality ratio (SMR) gradients derived for suicide and coronary heart disease (CHD) in 19992001 (England as standard).
The ratio of the SMR in the top quintile to that in the lowest quintile for each index is shown in Table 1 for males and females. Comparisons for suicide show the clearest impact of fragmentation for females, and the clearest impact of deprivation for males. For CHD deaths there is a strong deprivation effect but no fragmentation effect. Such contrasts in impact suggest the constructs are, at least at this broad spatial scale, largely independent and also that fragmentation (in the ecological sense) may have limited relevance to some major causes of mortality.
Ideally a deprivation measure would include small area income levels and a fragmentation measure would include measures of community ties (e.g. membership of local networks). However, operationalization of ecological constructs depends on rather imperfect indicators and is governed, in the UK at least, by what is recorded in Censuses. An interesting possibility is that the UK 2001 Census has included religion and one might add absence of religious affiliation to a redefined fragmentation index. There are plans to include proxies for social capital (neighbourhood ties) in the General Household Survey in the UK. Censuses (or indeed population registers in Scandinavian countries) offer other opportunities for index construction in terms of what they record.
In addition, a range of technical issues are also raised by such studies: for example, are the effects of area deprivation or fragmentation spatially constant?13 Does one convert indicators to constructs by factor analysis or by totalling z scores? Are there non-linear effects of deprivation or fragmentation? Spatial correlation in area effects in multi-level studies is also a relatively neglected area. Much research remains to be done in both index construction and technique.
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