Trend interpretation and classification

In addition to yearly indices, it is relevant to assess the trend over the whole study period. This trend is the slope of the regression line through the logarithm of the indices. This slope, the standard trend estimate used in scientific papers, is called the additive trend in TRIM.

Besides, TRIM calculates the multiplicative trend, which is easier to interpret for non-specialists. This multiplicative trend reflects the changes in terms of the average percentage change per year. If this trend is equal to 1, then there is no change. If the trend is, e.g. 1.08, then there is an increase of 8% per year. This means: in year 2, the index value will be 1.08, in year 3 1.08 x 1.08 = 1.17 etc. If the trend is, e.g. 0.93, then there is a decrease of 7% per year.

Both trend estimates are different descriptions of the same estimates: the additive parameter is the multiplicative parameter’s natural logarithm.

The multiplicative trend estimate (trend value) in TRIM is converted into one of the following categories to facilitate its interpretation further. The category is determined by the value of the multiplicative trend itself and its uncertainty. Here it’s 95% confidence interval (= trend estimate +/- 1.96 times the standard error of the trend).

• Strong increase – a trend slope of >1.05 (an increase of more than 5% per year), with the lower confidence limit of the slope >1.05
• Moderate increase – a trend slope between 1.00 and ≤1.05 (an increase of no more than 5% per year), with the lower confidence limit of the slope between 1.00 and 1.05
• Stable – a trend slope where the confidence intervals overlap 1 (no significant change), with the lower confidence limit of change >0.95 and upper confidence limit of change <1.05
• Uncertain – a trend slope where the confidence intervals overlap 1 (no significant change), with the lower confidence limit of change <0.95 and/or the upper confidence limit of change >1.05
• Moderate decline – a trend slope of ≥0.95 and 1.00 (a decline of no more than 5% per year), with the upper confidence limit of the slope between 0.95 and 1.00
• Steep decline – a trend slope of <0.95 (a decline of more than 5% per year), with the upper confidence limit of the slope <0.95

See the TRIM manual (Pannekoek & Van Strien, 2001) for technical details on the computation.