The method TRIM consists of converting time series and analysing trends. It is very suitable for analysis of time series of counts with many missing values. The output from TRIM is used for compiling indicators.
Converting time series
As actual numbers of animals and plants are often unknown, time series are converted to index numbers, The base year for these indices is (usually) the first year the monitoring network is operational; this year is set at 100. The index values make it possible to see very quickly how percentages change with respect to the base year, and to compare the changes for various species. In principle the series are corrected for possible distortion caused by under or over sampling of certain regions and vegetation types (e.g. woodland, heath land, dunes, marshland, etc.) If observations are missing, the TRIM estimates the missing values on the basis of changes observed on plots that were monitored. This means that when a new year is added the index figures on previous years may change.
Furthermore, trends are established over a numbers of years. TRIM expresses whether a species has increased or decreased significantly, has remained stable or if the change is uncertain. In this respect, stable means that a species has not actually increased or decreased. In StatLine the trend development is included in the explanatory note on the species concerned.
The index figures in turn are used to compile ecological indicators. Data on a number of separate species are taken to compile these aggregate figures. Statistics Netherlands’ publications include the geometric mean of indices of a number of species. Such an index is called a species group trend index (SGT). The reports of the Milieu- en Natuurplanbureau (Environment and Nature planning agency) also include other ecological indicators based on data from the monitoring. The compilation of SGTs per type of vegetation is based on a selection of characteristic species from the species group.