Another name for opportunity sampling is convenience sampling. While creating a sample group helps to minimize the time and expense of studying every member of a population, sometimes a perfect sample will still involve too many people across too diverse of an area, thus affecting or even biasing the potential results of the study in a way you don't want.

An example of this problem would be studying senior citizens enrolled in college. Because it's likely in the thousands, the total population of senior citizens in college is too large to study. If you create a sample of seniors enrolled in every college in America, you still have an unmanageable sample of far too many people in an area that's far too diverse to contact effectively. However, the colleges in your local area or in your state can provide you convenient access, or opportunities, to enough senior citizen college students to create a reasonably representative sample.

While there are some problems with this method, such as regional bias, opportunity sampling can often prepare a preliminary study on which to base larger, better funded, research projects.