Environmental contaminants are typically found in minute quantities. Careful laboratory analyses are required to detect concentrations in the parts per million or parts per billion. A small error can give an inaccurate impression of risk, or misleading information about trends over time. Rigorous testing of the laboratories involved is the only way to determine if reported results are reliable. The most reliable approach is intercomparison, in which various laboratories analyze identical samples. The results are then compared to assess quality of results and to determine if there are any systematic errors