ICP Waters has a programme manual that contains information on site selection, recommended chemical analysis and other items that are important to keep up a high quality of the monitoring programme. Read more about the manual here. Each country contributes data from national monitoring programmes. Between-laboratory quality control is necessary to assure clear identification and control of the bias between analyses carried out by individual participants of the ICP Waters Programme. Such biases may arise by use of different analytical methods, errors in the laboratory calibration solutions, or through inadequate within-laboratory control. The number of participating laboratories in the chemical intercomparison has increased from 9 in 1987 to 72 laboratories in 28 countries in the 21st chemical intercomparison in 2007. The intercomparison confirms that the results are generally compatible between laboratories. The purpose of the annual biological intercalibration is to evaluate the quality of the taxonomic work on the biological material delivered to the Programme centre. The 11th intercalibration was conducted in 2007. Twelve laboratories participate on a regular basis. Read more about the annual chemical intercomparison and biological intercalibration here.