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A selected subset of a population that can be random or nonrandom and representative or nonrepresentative. (Adapted from: Centers for Disease Control and Prevention 2014)

secular trends

Non-cyclical changes that occur over time. (Adapted from: Centers for Disease Control and Prevention 2014)


The unconsolidated grains of minerals, organic matter, or preexisting rocks, that can be transported by water, ice, or wind and deposited. (Source: Schlumberger 2019)

selection bias

Systematic difference in the enrollment of participants in a study that leads to an incorrect result (e.g., risk ratio or odds ratio) or inference. (Adapted from: Centers for Disease Control and Prevention 2014)


The ability of a test, case definition, or surveillance system to identify true cases; the proportion of people with a health condition (or the proportion of outbreaks) that are identified by a screening test or case definition (or surveillance system). (Adapted from: Centers for Disease Control and Prevention 2014)


A cylindrical or spherical vessel used to separate oil, gas, and water from the total fluid stream produced by a well. Separators can be either horizontal or vertical. Separators can be classified into two-phase and three-phase separators (commonly called free-water knockout). Gravity segregation is the main force that accomplishes the separation, which means the heaviest fluid settles to the bottom and the lightest fluid rises to the top. (Source: Schlumberger 2019)


A fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers, it is the most abundant sedimentary rock. Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter. (Source: Schlumberger 2019)

shale gas

Natural gas that can be generated and trapped within shale units. (Adapted from: Schlumberger 2019)

source rock

A rock rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon, although a rich source rock might have as much as 10% organic matter. (Source: Schlumberger 2019)


To start the well drilling process by removing rock, dirt, and other sedimentary material with the drill bit. (Source: Schlumberger 2019)

standard error (of the mean)

The standard deviation of a theoretical distribution of sample means of a variable around the true population mean of that variable. Standard error is computed as the standard deviation of the variable divided by the square root of the sample size. (Adapted from: Centers for Disease Control and Prevention 2014)


Layers of sedimentary rock. (Source: Schlumberger 2019)

stray gas

Gas that migrates from its usual geological location into aquifers or the vadose zone (the zone between the groundwater and the surface) within the shallow subsurface. (Source: Groundwater Protection Council 2012)


Without apparent symptoms. (Adapted from: Centers for Disease Control and Prevention 2014)


A systematic canvassing of persons to collect information, often from a representative sample of the population. (Adapted from: Centers for Disease Control and Prevention 2014)

synthetic control method

A statistical tool used to estimate what would have happened to the exposed group had they not been exposed, while controlling for potential confounders that may change over time. (Adapted from: Bouttell et al. 2018)