Consecutive Non-Significant Segments

Sometimes, the APC for one segment is significantly different from zero, but when an extra joinpoint in the segment is determined by the Joinpoint software, neither APCs for the two consecutive segments are significant. Why?

Answer:

First, the test of APC is based on asymptotic t-test and the number of joinpoints is determined by permutation test, which is more reliable. When a segment is divided into two segments by a joinpoint, there are less data points in each segment, which usually decreases the power of each test. An omnibus test, which combines both segments, is a more powerful test to detect the overall trend for both segments.