Interpretation and Role
The trend test will have higher power than the chi-squared test when the suspected trend is correct, but the ability to detect unsuspected trends is sacrificed. This is an example of a general technique of directing hypothesis tests toward narrow alternatives. The trend test exploits the suspected effect direction to increase power, but this does not affect the sampling distribution of the test statistic under the null hypothesis. Thus, the suspected trend in effects is not an assumption that must hold in order for the test results to be meaningful.
Read more about this topic: Cochran-Armitage Test For Trend
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