Teun Remmers

140 | Chapter 8 data may depend from one municipality to another. As variability in the quality and data- structure of GIS data hampers between-study comparisons, researchers are encouraged to provide insight in the various levels of detail underlying their spatial analyses. The present study also aimed to investigate the influence of children's exposure to objectively-measured multi-place environments on physical activity. Systematic reviews investigating relationships between the environment and PA urged for objective measurements of both PA and the built environment (53, 54). Consequently, there has been an increase in studies combining accelerometer and GPS measurements, to investigate relationships between environment and behaviour using contemporaneous momentary designs. In this design, objective data on characteristics of the environment (based on the GPS location) and PA intensity is analysed contemporaneously (22, 32, 35, 55-61). In other epidemiological studies investigating for example air pollution, exposure can be defined as the cumulative time within 50 meters from a certain pollutant regardless of the type of behaviour performed (62). In contrast, PA is a complex interplay between spontaneous and planned behaviour; involving memory of PA facilities, time- and capacity constraints, social interactions, and compensation mechanisms. Therefore, children's actual exposure to PA facilities may not be as time and location dependent as these momentary analyses attempt to capture. Hence, studies using contemporaneous momentary designs should be aware of their limitations. For example, these designs may be especially vulnerable to selective daily mobility bias, which may occur when environments are deliberately visited for PA participation (e.g. sports grounds). In this way, it is not the environment that influences PA, but a participant's pre-conceived choice that biases the PA-environment relationship (27, 29). In contrast to the contemporaneous momentary design, the present study identified accessibility-measures from theory-based spatial anchor points (i.e. participant's residence, primary school and daily transport route). However, as illustrated in Figure 2, our multi-place environments (i.e. school, home and daily transport environment) aggregate to the same neighbourhood environment if children live closer to school. The percentage of 'shared' neighbourhood environment (and thus the added value of using this multi-place environment instead of regular residential or school environments) depends on the distance children reside from their school and the choice of the buffer size. This means that future studies are encouraged to make informed decisions about children's daily exposure environments, based on distances between school and homes, and knowledge of potential other frequently visited anchor points. For example, some studies incorporated participant's own perceptions of their daily mobility environments (63). Moreover, multi-place environments can also be merely considered as children's cognitive activity space in the sense that they have knowledge about relative location of potential PA opportunities from both the school and residential anchor points (22, 27, 29). Consequently, although our exact specification of the buffer areas may thus be under debate, we believe we have used an optimized methodology of assessing objectively measured environmental exposure assessment.