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and equal chance of being selected. The chance is known because the total number of people in the
sampling frame is known. For example, if every other person from the sampling frame were chosen, each
person would have a 50 percent chance of being selected.
A nonprobability sample is any type of sample that’s not drawn in a systematic way. So the chances of
each would-be participant being selected can’t be known. A convenience sample is one type of
nonprobability sample. It is a sample a researcher draws because it’s readily available and convenient to
do so. Surveying people on the street as they pass by is an example of a convenience sample. The question
is, are these people representative of the target market?
For example, suppose a grocery store needed to quickly conduct some research on shoppers to get ready
for an upcoming promotion. Now suppose that the researcher assigned to the project showed up between
the hours of 10 a.m. and 12 p.m. on a weekday and surveyed as many shoppers as possible. The problem is
that the shoppers wouldn’t be representative of the store’s entire target market. What about commuters
who stop at the store before and after work? Their views wouldn’t be represented. Neither would people
who work the night shift or shop at odd hours. As a result, there would be a lot of room for sampling error
in this study. For this reason, studies that use nonprobability samples aren’t considered as accurate as
studies that use probability samples. Nonprobability samples are more often used in exploratory research.
Lastly, the size of the sample has an effect on the amount of sampling error. Larger samples generally
produce more accurate results. The larger your sample is, the more data you will have, which will give you
a more complete picture of what you’re studying. However, the more people surveyed or studied, the more
costly the research becomes.
Statistics can be used to determine a sample’s optimal size. If you take a marketing research or statistics
class, you will learn more about how to determine the optimal size.
Of course, if you hire a marketing research company, much of this work will be taken care of for you.
Many marketing research companies maintain panels of prescreened people they draw upon for samples.