Delivering a Healthy WA
Western Australian Cancer Registry

Statistical methods

Rates

Rates are calculated separately for males and females, expressed as events (diagnoses or deaths) per 100,000 person-years.

  • Age-specific rates (ASPR) are based on five-year age groups and are calculated by dividing the numbers of cases by the population of the same sex and age group.
  • Age-standardised rates (ASR in Tables) are calculated by the direct method, as a summation of weighted age-specific rates. Tables show the 95% confidence interval (c.i.) for ASRs.
  • When a subset of age groups (e.g. 15-39 years) is considered, the term age-adjusted rate is used instead of ASR, as standardisation has considered only some age groups, for both cases and population.
  • Comparison of rates between different areas may be done using indirect standardization. In this process, for example, the State population and age-specific rates are used to calculate an expected number of cases in different areas, based on their populations; the observed and expected numbers are compared. The Standardized Incidence Rate Ratio (SIRR), derived from indirect standardization, is the ratio of incidence in one area compared to that in a reference area, usually the whole State. An SIRR of 1 indicates that rates are the same.

The World Standard Population 1960 (Segi M, 1960) remains in routine use for ASR calculation, as in most cancer registries worldwide. However in some tables a second ASR and 95% c.i. are shown, labelled "ASR2". These use the Australian (2001) population standard (external link). These ASRs are usually quite different, and comparisons need to take note of which "standard" is being used.

"Death certificate only" cancers

Records are created for persons with cancer for which no information other than a death certificate is available. Efforts are then made to obtain independent verification of tumour details. If no supporting information can be obtained after research these are treated in subsequent reports as "death certificate only" (DCO) tumours.

Supporting information is eventually obtained for the vast majority of these cases, and only 0.5–1% of tumour records remain in this category. For example, there were 24 DCO cancers recorded for 2005, representing only 0.26% of all cancers.

» Back to definitions of terms used