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.

In addition, TRIM calculates the multiplicative trend, which is easier to interpret for laymen. This multiplicative trend reflects the changes in terms of 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 natural logarithm of the multiplicative parameter.

The multiplicative trend estimate (trend value) in TRIM is converted into one of the following categories to facilitate its interpretation further. The category is not only determined by the value of the multiplicative trend itself, but also by its uncertainty, here its 95% confidence interval (= trend estimate +/- 1.96 times the standard error of the trend).

  • Strong increase – increase significantly more than 5% per year (5% would mean a doubling in abundance within 15 years). Criterion: lower limit of confidence interval > 1.05.
  • Moderate increase – significant increase, but not significantly more than 5% per year. Criterion: 1.00 < lower limit of confidence interval < 1.05.
  • Stable – no significant increase or decline, and most probable trends are less than 5% per year. Criterion: confidence interval encloses 1.00 but lower limit > 0.95 and upper limit < 1.05.
  • Uncertain – no significant increase or decline, and unlikely trends are less than 5% per year. Criterion: confidence interval encloses 1.00 but lower limit < 0.95 or upper limit > 1.05.
  • Moderate decline – significant decline, but not significantly more than 5% per year. Criterion: 0.95 < upper limit of confidence interval < 1.00.
  • Steep decline – decline significantly more than 5% per year (5% would mean a halving in abundance within 15 years). Criterion: upper limit of confidence interval < 0.95.

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