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Figure 2 | International Journal of Behavioral Nutrition and Physical Activity

Figure 2

From: Categorisation of built environment characteristics: the trouble with tertiles

Figure 2

Comparison of approaches for estimating non-linear relationships using the SESAW study [ 3 ]. (a) Comparison of a simple linear model, fractional polynomial (of which the best fitting was equivalent to the simple linear model), linear splines, tertiles and a non-parametric smoother (see Table 1 for the respective AICs to assess model comparison). (b) As in Figure 2(a) with an extension to the y-axis to show the complete range of BMI and the observed data plotted (n = 1462 points). We see visually the result of comparing the AICs in Table 1 that due to the large variance in BMI scores there is no evidence for anything more complicated than a simple linear model. Further, there is nothing statistically to choose between the linear and tertile fits. However, the linear model has the benefit of not being data-dependent.

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