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observational study

A study in which the investigator observes rather than influences exposure and disease among participants. Case-control and cohort studies are observational studies. (Adapted from: Centers for Disease Control and Prevention 2014)

odds ratio

A measure of association used in comparative studies, particularly case-control studies, that quantifies the association between an exposure and a health outcome; also called the cross-product ratio. (Adapted from: Centers for Disease Control and Prevention 2014)


A mixture of hydrocarbons usually existing in the liquid state in natural underground pools or reservoirs. Gas is often found in association with oil. (Source: U.S. Energy Information Administration 2019)

oil or gas field

The surface area overlying an oil or gas reservoir or reservoirs. Commonly, the term includes not only the surface area but also the reservoir, wells, and production equipment. (Source: OSHA 2001)

oil well

A producing well with oil as its primary commercial product. Oil wells almost always produce some gas and frequently produce water. Most oil wells eventually produce mostly gas or water. (Source: Schlumberger 2019)

orphaned well

Orphaned wells have no known or solvent owner and may or may not be capable of further production. (Source: Interstate Oil and Gas Compact Commission 2009)


Any or all of the possible results that can stem from exposure to a causal factor or from preventive or therapeutic interventions; all identified changes in health status that result from the handling of a health problem. (Adapted from: Centers for Disease Control and Prevention 2014)