The highest dosage administered that does not produce toxic effects. The NOAEL will depend on how closely dosages are spaced (lowest-observed-adverse-effect level and no-observed-effect level) and the number of animals examined. The ultimate objective is usually to determine not the "safe" dosage in laboratory animals but the "safe" dosage for humans. Therefore, the extrapolation most often required of toxicologists is from high-dosage studies in laboratory animals to low doses in humans. (Casarett and Doull's Toxicology: The Basic Science of Poisons, 4th ed).