Which year should be chosen as the base time year?
Often the first year of the time series is chosen as the base year in order to test the changes against the base year. In case the species is not present in the first year, another year should be selected. A different strategy might be to select the base year in order to minimise the standard errors of the indices. By definition the standard errors of the index of the base year are zero and all errors associated with the base year index are transferred to the indices of the other years. Therefore, the standard errors of the indices may vary with the choice of the base year.