In trend analysis, what does a p-value indicate?

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In trend analysis, the p-value expresses the probability of obtaining a result as extreme as the one observed (or more extreme) under the null hypothesis that there is no trend. In other words, it’s evaluated assuming no trend exists, not assuming a trend does. A small p-value suggests the data are unlikely if there is no trend, providing evidence against the no-trend assumption. A large p-value means there isn’t enough evidence to claim a trend. The p-value does not measure the likelihood that a trend exists, nor its magnitude (that’s given by the slope estimate), nor the precision of the trend estimate (which is reflected in the confidence interval).