This appendix provides information on geographical patterns and trends in overall cancer survival in England, to complement the Statistical Bulletin published on 19 December 2012: “Index of Cancer Survival for Primary Care Trusts in England: Patients diagnosed 1996–2010 and followed up to 2011”.
The appendix contains a set of smoothed maps and funnel plots. Smoothed maps and funnel plots have different purposes. Smoothed maps provide a strategic overview of geographic patterns in survival across the country, and how they change over time. Funnel plots show how the values of the cancer survival index in each Primary Care Trust (PCT) differ from the national average.
Smoothed maps of the survival index were constructed for each year in the 15-year period 1996–2010 to provide a visual impression of trends in the geographic pattern of one-year survival for all cancers combined across England. Three examples of the one-year survival index are shown for the selected years 1996, 2003 and 2010: for patients aged 15–99 (Figures 1a–c), for patients aged 55–64 years (Figures 2a–c) and for patients aged 75–99 years (Figures 3a–c).
In these maps, the PCT boundaries are not shown. The survival index at each point on the map is weighted by all other estimates within a smoothing circle around it, with weights inversely proportional to their distance. The circles on the maps represent the PCTs with the largest populations, which have statistically stable survival estimates and are not included in the smoothing process. The point is to provide a “bird’s-eye” view of geographic variation in survival across the country, after elimination of random local fluctuations.
Funnel plots show how much a particular survival index deviates from the overall survival index for that year in England as a whole (the horizontal line in each plot), given the statistical precision of the index for that PCT. The precision is the inverse of the variance of the survival index, derived from the model. Funnel plots of the one-year survival index for all cancers combined are presented in Figures 4a–c for all adult patients (aged 15–99 years), Figures 5a–c for patients aged 55–64 years and Figures 6a–c for patients aged 75–99 years. Each data point represents one PCT.
The two ‘control limits’, in the shape of a funnel around the horizontal line, represent approximately two (95% control limits) and three (99.8% control limits) standard deviations, respectively, above or below the national average. Data points that lie inside the control limits may be considered as within the range of geographical variation that could be expected by chance. Values of the survival index that fell below the control limits for 1996 are marked with a red dot. They represent the PCTs for which the cancer survival index was significantly lower than the national average for patients diagnosed in 1996, after taking account of differences between PCTs in the statistical precision of the index. These PCTs with ‘outlier’ values of the index for 1996 are also marked in red in the funnel plots for later years (2003 and 2010), to provide a visual indication of the extent to which the survival index in those PCTs has improved or remained consistently low over time. It should be noted that these ‘outlier’ values are not just the lowest values that would be obtained from a simple ranked list. Some of the lowest values of the index are still within the range of variation that could be expected by chance (that is, inside the control limits). These are generally for the smallest PCTs, in which the index has lower precision.
We gratefully acknowledge Mr Toni Patama and Professor Eero Pukkala at the Finnish Cancer Registry, who kindly provided the software with which the smoothed maps were produced, and who assisted in their production.
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