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Comparing the effects of infrastructure on bicycling injury at intersections and non

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Vulnerabilities of the case crossover method as applied to transportation injuries and traffic engineering problems

Canada Goose Parka The case crossover method in its familiar application is to look for factors that recur when cases occur, for individuals crossing exposure to them as examined over canada goose clearance a time interval. This study [1 3] applies the method in a different way, the exposures being examined over a spatial route, with neither the identified factors nor the various routes being independent of the highly constrained urban geographies of the se. Canada Goose Parka

Canada Goose Outlet The case crossover method in its familiar application is to look for factors that recur when cases occur, for individuals crossing exposure to them as examined over a time interval. This study [1 3] applies the method in a different way, the exposures Canada Goose Outlet being examined over a spatial route, with neither the Canada Goose Coats On Sale identified factors nor the various routes being independent of the highly constrained urban geographies of the settings. Thus in addition to all the familiar vulnerabilities of the case crossover method [4 9] some of which require translation to the new context this application brings new problems of its own. It is not possible to note even most of just the Canada Goose Online major ones in this correspondence, and further criticism may be found elsewhere [10 12]. Canada Goose Outlet

1. Control site selection bias. Contrary to [2], such comparisons only make sense if the control locations match the case locations by intersection status, which often they do not. This was necessary for about 70% canada goose coats of the cases at intersections, and 30% at non intersections.

canada goose black friday sale In these instances, selection of control canada goose uk shop intersections is dependent on their spatial distribution along the route, but indifferent to their widths. This biases their selection in favour of smaller intersections associated to longer non intersection segments. cheap canada goose uk Likewise, the selection Canada Goose Parka of non intersection control sites is disproportionately biased in favour of whatever are adjacent larger intersections. canada goose black friday sale

cheap Canada Goose For example, over a route whose length is 30% intersections, buy canada goose jacket cheap 70% non intersections, beginning at 0 and having terminated at 1, with intersections between Canada Goose online 0 and 0.05, 0.5 canadian goose jacket and 0.6, and 0.85 to 1, the probabilities of choosing the three intersections as control sites should occur in ratios canada goose black friday sale of 1:2:3. Maclure [4] has discussed the potentially large biases in relative risk estimates that can result from not taking intersection widths into account. cheap Canada Goose

canada goose coats on sale There is already selection bias before this stage. For example, consider canada goose https://www.canadagoosestorevip.com coats on sale a route with uk canada goose outlet no intersections, having a bicycle specific facility in the first and last thirds only. Suppose injury events occur at Canada Goose Jackets random canada goose along this route. They should therefore occur in facilities and canada goose uk outlet non facilities in proportions of 2:1, and likewise so should the selection of control sites. canada goose coats on sale

Canada Goose sale Such problems have been discussed extensively in the meteorological literature on case crossover studies [7 9], and by Maclure in the epidemiological literature [4]. Canada Goose sale

buy canada goose jacket cheap There is still another potential Canada Goose sale randomisation failure at this level of selection to consider. The standard deviation of the uniform distribution on [0, 1] is almost one third (1/[2SQRT(3)]). For individual runs of only 801 in length (for non intersections), or 272 (for intersections), this can easily result in quintiles being out of balance by plus or minus 10 to 25%, which can again skew the estimates. (Thus the reader wishing to closely check by simulation the probability calculations given above should use a much larger n, such as on the order of 10^5.) buy canada goose jacket cheap

2. Anomalous or internally inconsistent results. They suggest their finding may not be generalisable. But they do not explain why their method should have led to a non generalisable result. Instead it ranges over entirely different locations, which coincidentally may have different values of the independent variable. Yet cyclists do not choose their routes at random, and many canada goose outlet routes may share intersections and links.

In most cities there are inherently hazardous locations that attract bicycle traffic because they are in some way inevitable, such as for being the only way to access a bridge. Thus even if control site canada goose factory sale selection within routes had been correctly randomised, this would not balance the bias across routes.