It is critical to the success of a stratified design that each strata is homogeneous within, but distinct from, others. As an example, suppose an inventory of an owl species is planned for a forested valley which contains three distinct age-classes of forest in equal quantities. If the owl in question has a proven affinity for mature forest, it may be unnecessary and impractical to survey the entire valley with equal effort. A more successful study might begin by stratifying the forest by age class prior to sampling, and focusing more effort on to those strata where owl numbers are expected to be highest. Rather than spending large amounts of time and money recording null data for early age classes, a stratified sampling design allows surveyors to maximize the number of observations in areas where variability is highest.
Most ecological surveys have some degree of stratification incorporated into the design.
A stratified sampling design can work as a framework for either a systematic or random design. This is especially true if the measurements within the strata are homogeneous. Cost per observation is normally reduced. If estimates of population parameters are desired for subgroups of the population, separate estimates can be obtained for each. Stratification is encouraged when doing species inventory in British Columbia.