EnviroInsite uses the Sen Slope method described in EPA/240/B-06/003 for calculating the slope of trend lines. The Sen Slope is a non-parametric method calculated as the median of the slope between all measurement pairs. The trend line is set to pass through a point set to the median of the sample dates and the median of the plotted values.
Why not use linear regression as a measure of trend? The calculated slope from linear regression is a robust method for calculating trends when the data conforms to the method's underlying assumptions. For instance, the presence of outliers can make the calculated slope of a regression line unrepresentative of the overall temporal trends. Take a look at how outliers can throw off linear regressions in the plot at the Wikipedia article on Sen Slope.
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