[1] For example, if doing a study of fitness where age or gender was expected to influence the outcomes, participants could be stratified into groups by the confounding variable.
[1] Proportionate stratified sampling involves selecting participants from each stratum in proportions that match the general population.
Alternatively, disproportionate sampling can be used when the strata being compared differ greatly in size, as this allows for minorities to be sufficiently represented.
[1] Stratification is used in quota sampling, a non-random method in which the researcher identifies strata of the population and pre-determines how many participants are needed from each stratum.
[1] This is considered a better method than convenience sampling, as it attempts to ensure different strata are properly represented.