Reply to investigating changes over time in socioeconomic gaps in cancer survival: does choice of approach matter?
Authors: G. Lyratzopoulos, B. Rachet, M.P. Coleman
Journal: Annals of Oncology (2011)

We thank Blakely et al. for their comments. They raise two important and recurrent issues in epidemiology, namely the choice of outcome measure and whether it is measured on an absolute or relative scale. We agree that both issues can influence measurement and interpretation of trends in socioeconomic inequality in cancer survival. In general, the relative scale is better suited to measures such as a death rate, with an open-ended range of values, while the absolute scale is preferable for measures such as survival, with a finite range (e.g. 0%–100%). To show how absolute or relative measures influence the socioeconomic deprivation gap in survival, we tabulate both the observed survival values for 1973 and 2004 and a hypothetical scenario in which we subtract 45% (breast) or add 40% (rectum) to the observed values (Table 1). For breast cancer, both absolute and relative measures show a reduction of the deprivation gap in survival with time, but on the relative scale, the reduction depends heavily on the actual values. For rectal cancer, the absolute gap widens from -6% to -12% in both scenarios, but the relative gap changes very little (0.82 versus 0.80) with the original values or widens slightly (0.92 versus 0.88) with the hypothetical data. This example shows that use of the relative scale distorts the contrast when the survival values are in the tails of the survival distribution, close to 0% or 100%. The absolute and relative scales only produce similar contrasts when the survival values are closer to 50%.Table 1Relative survival (%) in deprived and affluent patients, with absolute and relative measures of the deprivation gap in survival: breast and rectal cancer, England and Wales, 1973 and 2004Relative survival (%)Deprivation gap in survivalDeprivedAffluentAbsoluteRelativeValuesBreast (women)Observed19735060-100.8320048086.5-6.50.92Hypothetical1973515-100.3320043541.5-6.50.83RectumObserved19732733-60.8220044759-120.80Hypothetical19736773-60.9220048799-120.88 Open table in a new tab We agree that useful information can be added by outcome measures on the mortality scale, such as the excess hazard of death, or the excess hazard ratio between two groups. These measures can only be derived from relative survival. As with survival, they vary widely with time since diagnosis and so they should be reported for specific time intervals since diagnosis. This enables us to identify the time since diagnosis when the differences in survival arise [1.Jeffreys M. Rachet B. McDowell S. et al.Survival from rectal and anal cancers in England and Wales, 1986-2001.Eur J Cancer. 2006; 42: 1434-1440Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar, 2.Woods L.M. Rachet B. O'Connell D.L. et al.Large differences in patterns of breast cancer survival between Australia and England: a comparative study using cancer registry data.Int J Cancer. 2009; 124: 2391-2399Crossref PubMed Scopus (17) Google Scholar]. Another mortality outcome derived from relative survival is the cumulative number of deaths within, say, 5 years of diagnosis that would be avoidable if deprived patients had the same relative survival at 5 years as affluent patients [3.Abdel-Rahman M.A. Stockton D.L. Rachet B. et al.What if cancer survival in Britain were the same as in Europe: how many deaths are avoidable?.Br J Cancer. 2009; 101: 115-124Crossref PubMed Scopus (135) Google Scholar]. If, for a given cancer, the incidence rate is stable and survival is improving (a fairly common situation), but the absolute difference in survival between deprivation groups is unchanged, then the 'number' of avoidable deaths will fall simply because the overall number of deaths will have fallen, indicating a decline in the public health impact of these inequalities. Both relative and absolute measures can therefore be used to measure inequality in cancer outcomes. When examining survival, however, we feel that it is usually easier to interpret absolute differences between population groups as measures of improvement in cancer control and for the purpose of monitoring inequalities in cancer outcomes. The authors declare no conflicts of interest.