Partitioning linear trends in age-adjusted rates

Objective: Surveillance of chronic diseases includes monitoring trends in a ge-adjusted rates in the general population. Statistics that are calculated to describe and compare trends include the annual percent change and the p ercent change for a specified time period. However, it is also of interest to determine the contribution specific diseases make to an overall trend in order to better understand the impact of interventions and changes in the prevalence of risk factors. The objective here is to provide a method for p artitioning a linear trend in age-adjusted rates into disease-specific comp onents. Methods: The method presented is based on linear regression. The decreasing trend in age-adjusted cancer mortality rates for the total United States d uring the period 1991-96 is analyzed to illustrate the method. Results: Trends in mortality for cancers of the colon/rectum, breast, lung/ bronchus, and prostate are found to be responsible for 75% of the decreasin g trend in cancer mortality. Conclusions: It is possible to partition an overall trend in age-adjusted r ates under the assumption that it and the trends for all mutually exclusive and exhaustive subgroups of interest are linear.