De Multi-Species Indicator (MSI) is the geometric average of the indices in a set of (either pre-selected or unselected) species. For the species in the indicator, counts in a particular year (the base year) are set as 100 and the counts in all other years are then indexed to the base year. For each year, the geometric average of the indices is calculated. This is the average of the natural logarithms of the indices after applying inverse transformation. By taking the geometric average, a doubling in the population of one particular species and a halving in the population of another species carry equal weight. The doubled and the halved population offset each other, thereby rendering the MSI unchanged.
Furthermore, the geometric average has several other desirable mathematical properties for biodiversity indicators (Van Strien et al. 2012). The MSI is used extensively, e.g. in the form of the Living Planet Index (LPI). CBS has developed software which allows for statistical trend assessment in the MSI (Soldaat et al. 2017).