Joinpoint APCs not matching SEER*Stat APCs

When I fit a Joinpoint model to find the rate trend, why doesn't the APC for one of the segments match the APC found by SEER*Stat for the corresponding segment?

Answer:

The Joinpoint model assumes that the trend is continuous at the joinpoint, whereas the APC for the corresponding segment calculated by SEER*Stat does not impose the continuity constraint with the consecutive segment. Hence, when there are one or more joinpoints, the APCs from Joinpoint and from SEER*Stat are different, but they should be close to each other. When there are zero joinpoints, the APCs from Joinpoint and SEER*Stat will be the same, except for differences due to the precision level of the data.