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Glossary Acellular or hypocellular specimen: A specimen that contains no cells or has fewer cells than normal, such as when using a f~ne-needle aspiration. Activity: The amount of a radioactive nuclide in a particular energy state at a given time. Units of activity are becquere! (Bq) and curie (Ci); ~ Ci = 3.7 x 10~° disintegrations per second, and ~ Bq = ~ disintegration per second. Acute release: An instantaneous release into the environment, similar to a puff of radioactivity. Ascertainment bias: Systematic error due to a difference in characteristics between those who choose to participate in a study and those who do not. Attenuation': In statistics, a bias in a study in which the estimated strength of an observed dose response tends to be lower than the true dose response. Attenuation is expected in a dose-response study in which exposure is estimated with error under the classical measurement-error model. 200