Glossary

A naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible. Methane is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane, propane, butane, and pentane. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulfide. (Source: Schlumberger 2019)

Variables used to detect sources of bias or confounding in a study that are expected to be associated with the same unmeasured factors as the exposure of interest but not the outcome of interest. (Adapted from: Centers for Disease Control and Prevention 2014)

Variables used detect sources of bias or confounding in a study that are expected to be associated with the same unmeasured factors as the outcomes of interest but not the exposure of interest. (Adapted from: Lipsitch et al. 2010)

A type of information bias in which the proportion of subjects misclassified on exposure does not depend on disease status (case-control studies) or when the proportion of subjects misclassified on disease does not depend on exposure (cohort studies). (Adapted from: Rothman and Greenland 1998).

NORM is any terrestrial material (rock, soil, or water) that contains elements that emit radiation. TENORM is produced when activities such as those associated with oil and gas development concentrate or expose radioactive materials that occur naturally in ores, soils, water, or other natural materials. (Source: Babcock et al. 2015)

The supposition that two (or more) groups do not differ in the measure of interest (e.g., incidence or proportion exposed); the supposition that an exposure is not associated with the health condition under study, so that the risk ratio or odds ratio equals 1. The null hypothesis is used in conjunction with statistical testing. (Adapted from: Centers for Disease Control and Prevention 2014)