Progress in data quality reported as percentage of non tolerable results (by courtesy of Alfred Furst, see also http://www.waldwissen.net/themen/inventur_monitoring/oekosystemmonitoring/bfw_ringtest_2009_DE). Tolerable limits are set by ICP-forests (expert panel on foliage and litterfall) and have been tightened during the course of time. For example, for N the limit has been 10% deviation from the mean in the ring test from 1999 onwards (for other elements see Furst, 2008). See Table IV for details about the tests.