Territorial units

Territorial units are mutually exclusive “entities that have a social consistence

independent of a specific individual” (Chaix et al. 2009, p. 1306). Administrative and census areas are examples of territorial units. Territorial units have most often been used to represent residential neighbourhoods, but they can be used to represent context beyond the residential (e.g., Wong and Shaw 2011). Four common types of territorial units are described below.

2.4.1.1 Administrative units

Until recently, the majority of health research delineated neighbourhoods used pre- existing administrative units such as meshblocks, census tracts, enumeration districts and suburb boundaries. The use of administrative units to represent where

participants live is still popular, presumably because they are predefined, relatively simple to use, and readily available. In addition, and perhaps more importantly, secondary data sources, such as census data, are often defined at an administrative unit level, making it easy to include such data in research (Diez Roux 2007, Pickett and Pearl 2001).

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Examples of empirical studies that used administrative units to delineate the environment include Smith et al. (2008) who measured neighbourhood walkability within census tracts, and Morland et al. (2002) who measured food accessibility within census tracts.

2.4.1.2 Aggregated administrative units (zones)

To delineate boundaries using aggregated administrative units, small administrative units - typically the smallest available census area - are used as building blocks to form larger aggregated administrative units. Relatively few researchers have used aggregated administrative units, perhaps because it requires the additional step of creating new zones, which is not always straightforward.

Automated zone design techniques provide methods of aggregating administrative units. Cockings and Martin (2005), took enumeration districts - the smallest census unit in the UK - and designed a zoning system at different scales and aggregation levels in order to explore the relationship between neighbourhood deprivation and health in a UK county. Riva et al. (2009), designed zones for Montreal based on Canadian dissemination areas. They designed their zones to be homogenous over the exposure variable of ‘active living potential’.

While most researchers have used small administrative units as the

zone/neighbourhood building blocks, Sabel et al. (2013), recently developed an experimental automated zone design method, using a small tessellated cell as a basic building block, to create new synthetic neighbourhoods in France. They found correlations between asthma and deprivation were higher for their new zones than French census areas of a similar size. As a result, they argued that the careful

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construction of neighbourhoods can aid our understanding of relationships between the social and physical neighbourhood environment and health.

While some researchers have aggregated units based on homogeneity of a particular characteristic (Riva et al. 2009), others have argued against using homogeneity as a criteria to create zones (Chaix et al. 2009, Pickett and Pearl 2001, Ross et al. 2004). Although homogeneity may be relevant for defining sampling units or implementing interventions, it may not be relevant as the sole criteria with which to delineate areas an individual is exposed to. In other words, a zone/neighbourhood need not be homogenous to affect the lives of residents.

2.4.1.3 Buffered administrative units

The use of buffered administrative units is a response to boundary problems. Boundary problems – also called edge effects – are where study areas such as neighbourhoods are bounded by a discrete border, yet the spatial processes are not (Fotheringham and Rogerson 1993). For instance, if a participant lives on the edge of a neighbourhood, the characteristics of the adjacent neighbourhood may be more relevant than the characteristics of the neighbourhood the participant resides in. Boundary problems are particularly relevant for territorial units. Techniques to address them have long been discussed in geography, but are still new in health research.

Recently, Frank et al. (2012), have used buffered administrative units in a study of the environment and adolescents physical activity. In this study, a census block group buffered by a 0.25 mile radius was used as the residential neighbourhood.

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2.4.1.4 Natural neighbourhoods

Some researchers have suggested that neighbourhoods need to be ‘natural’, or ‘ecologically meaningful’ (Pickett and Pearl 2001). These terms describe functional neighbourhood units that are delineated to better represent the local-level activity spaces of individuals by ensuring they contain the appropriate composition of physical and social characteristics (Bissonnette et al. 2012).

Researchers have taken different approaches to the delineation of natural neighbourhoods. Some create ‘natural’ neighbourhoods by aggregating

administrative units based on homogeneity of variables (Parenteau and Sawada 2011). Natural neighbourhoods created in this way are the same as aggregated administrative units.

Another approach has been to create natural neighbourhoods by drawing on local knowledge. Ross et al. (2004), for example, have delineated natural neighbourhoods using a combination of housing district maps, historical documents, census data, local perceptions, and consultations with real estate agents. This type of approach may also end up creating neighbourhoods based on aggregated administrative units. A small number of studies have delineated natural neighbourhoods based on physical features of the environment, such as roads, rivers and areas of parkland (Stafford et al. 2008, Cutchin et al. 2011). Because physical features play an important role in daily life - for example, most people have to travel around a lake - it is likely that physical features are an important component in determining realistic neighbourhood boundaries. As such, a consideration of physical features might have the potential to improve boundary delineation; either alone, or in combination with other methods.

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