How to interpret the overall trend?
The overall trend is a descriptive parameter for the trend slope over the whole period studied. It is the slope of the regression line through the logarithm of the indices or time-totals. TRIM calculates four different trends: (1) the slope of the regression line based upon model indices, with intercept, (2) the slope of the regression line based upon imputed indices, with intercept, (3) the slope of the regression line based upon model indices, without intercept and (4) the slope of the regression line based upon imputed indices, without intercept. Without intercept means that the regression line is forced through the base time point. This is sensitive to the choice of the base time point and is questionable from an ecological point-of-view. Therefore, we recommend to use the overall trend estimate with intercept, and particularly recommend the estimate based upon imputed indices (the second one). No estimate of the intercept itself is given, because the intercept differs per site.
The overall trend can be interpreted in terms of significant decrease, stable population numbers etc. We have added a procedure to classify the trends in Appendix C of the TRIM manual. But other classifications are valid too, so we did not incorporate this in TRIM.