Cancer incidence rates and the problem of denominators - a new approach in Indian cancer registries.

In India, the national census provides population figures once every 10 years. However, since cancer incidence data provide various rates for five year age groups, the calculation of the relevant population estimates for a given year between any two-census years, serving as denominators, assumes importance. The Individual Exponential Growth Rate Method is in current use by various Indian cancer registries to estimate the population by five yearly age groups. Using the five yearly age group estimates by the same method, various rates like the Crude rate, Age Standardized Rate and Cumulative Rates, are reported in Cancer Incidence in Five Continents, Vol. VIII. However, this approach has been shown to suffer from bias and often results in sacrificing the overall growth rate and corrections become necessary in five yearly age group populations to maintain it. We here show that the proposed Difference Distribution Method is able to maintain both negative as well as positive growth in different five yearly age group populations. For population projections, this method scores over the Individual Exponential Growth Method, serving as a new methodology for population estimation by five yearly age groups for inter-census years for Indian cancer registries.