How is the AAPC computed?
AAPC is derived by first estimating the underlying joinpoint model that best fits the data. The accompanying figure shows the joinpoint model for prostate cancer incidence from 1975-2003 (from the 2005 data submission), which found joinpoints in 1988, 1992, and 1995. (This model is fit under the default joinpoint parameters). The AAPC over any fixed interval is a weighted average of the slope coefficients of the underlying joinpoint regression line with the weights equal to the length of each segment over the interval. The final step of the calculation transforms the weighted average of slope coefficients to an annual percent change. If we denote bis as the slope coefficients for each segment in the desired range of years, and the wis as the length of each segment runs in the range of years, then: APCi = { (Exp(bi) − 1) } × 100 and In the prostate cancer example, to compute the AAPC from 1994 to 2003, we first note that an APC of −10.7 runs for 1 year (with a slope coefficient of −0.11
