Toxic metals and organic compounds are commonly present at very low concentrations in water, sediments, soils, and rocks. Concentrations cannot be quantified with 99% certainty; if those chemicals are present the instrument cannot distinguish them from zero. Concentrations below laboratory reporting limits are censored because their values are unknown. Censored values can be 70-80% of the available date, a meaningful amount of valuable information.
Correct analysis of censored data is particularly important when performing an ecological risk analysis (ERA) as part of the CERCLA Superfund process. ERAs involve data summation and comparisons to a reference value. Incorrectly included censored data produce wrong results, potentially increasing expenses for responsible parties and ineffective restoration. Excluding censored values produces even greater errors.