Statistics in medicine
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Statistics in medicine · Apr 1996
Sorting variables for each case: a new algorithm to calculate injury severity score (ISS) using SPSS-PC.
One of the more often used measures of multiple injuries is the injury severity score (ISS). Determination of the ISS is based on the abbreviated injury scale (AIS). ⋯ The first three sorted AISs representing the three most severe injuries of a person are then used to calculate injury severity score (ISS). This algorithm should be useful for analyses of clusters of injuries especially when more patients have multiple injuries.
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We consider the problem of estimating the spatial variation in relative risks of two diseases, say, over a geographical region. Using an underlying Poisson point process model, we approach the problem as one of density ratio estimation implemented with a non-parametric kernel smoothing method. ⋯ We also propose a Monte Carlo test of the null hypothesis of constant risk over the whole region, to avoid possible over-interpretation of the estimated risk surface. We illustrate the capabilities of the methodology with two epidemiological examples.
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Statistics in medicine · Nov 1995
Spatial variation of natural radiation and childhood leukaemia incidence in Great Britain.
This paper describes an analysis of the geographical variation of childhood leukaemia incidence in Great Britain over a 15 year period in relation to natural radiation (gamma and radon). Data at the level of the 459 district level local authorities in England, Wales and regional districts in Scotland are analysed in two complementary ways: first, by Poisson regressions with the inclusion of environmental covariates and a smooth spatial structure; secondly, by a hierarchical Bayesian model in which extra-Poisson variability is modelled explicitly in terms of spatial and non-spatial components. From this analysis, we deduce a strong indication that a main part of the variability is accounted for by a local neighbourhood 'clustering' structure. ⋯ There is no consistent evidence of any association with radon levels. Indeed, in the Poisson regressions, a significant positive association was only observed for one 5-year period, a result which is not compatible with a stable environmental effect. Moreover, this positive association became clearly non-significant when over-dispersion relative to the Poisson distribution was taken into account.
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Statistics in medicine · Oct 1995
Re-evaluating design specifications of longitudinal clinical trials without unblinding when the key response is rate of change.
The design of clinical trials often requires knowledge of quantities such as between- and within subject variances about which only imprecise information exists. To provide assurance that the study has the desired power to detect a minimum clinically meaningful difference between treatment groups. Gould, Gould and Shih, and Shih have recommended obtaining relevant information from the trial at an interim stage without unblinding. ⋯ The proposed method aims to re-evaluate the sample size and study duration in a way that maintains the trial's blinding, using an EM algorithm. Simulation results show that the effect on type I error rate in negligible, but the potential gain in power can be substantial. The procedure is simple to use in practice, as it does not unblind patients' treatment identifications, and, since it does not unveil the relative efficacy of treatments, it fulfils the requirement of a valid 'administrative' (interim) analysis.
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Statistics in medicine · Mar 1995
Historical ArticleMeta-analysis: reconciling the results of independent studies.