As principal of a high school, you want to know if your students prefer Coca-Cola or Pepsi. You do not have time to ask all 5,000 students their preference, but the English teacher has offered to ask all of his students and let this sample decide for the school. This process is known as opportunity or convenience sampling and, as with all sampling methods, there are advantages and disadvantages.
Very Convenient Method
The main advantage of opportunity sampling is that it is an easy way to obtain a large amount of data in a relatively short time frame. Market researchers at the mall who ask you to fill out a survey are probably taking an opportunity sample; they are trying to get as many people as they can to complete their survey in the shortest amount of time possible.
Highly Practical Approach
Sometimes opportunity sampling is the only available method of data collection. If the study is preliminary and the data does not have to be exact, then this type of sampling is efficient. Because it takes less time, opportunity sampling is cheaper than other forms of sampling, which may be more representative of the general population.
Population Highly Similar
If you are sampling from a population that is very similar, then opportunity sampling may provide a sufficient estimate of the population. Experiments with memory or situations in nature are examples where all members of the population may be highly similar so the method of sample selection is not of paramount importance.
Introduces Selection Bias
The main disadvantage of opportunity sampling is selection bias. No one likes to be rejected; if approaching people to take a survey in the mall, a researcher is likely to pick people who make eye contact, smile, or give other nonverbal cues that they will likely consent to take the survey. Additionally, researchers are more likely to choose people similar to themselves both socially and culturally.These factors introduce a bias into the sample selection.
Sample Not Representative
Due to selection bias, the sample polled using the convenience method may not be representative of the entire population from which it is drawn. Students in the English class are all subject to similar interests based on their age group, friendships and shared experiences that may cause them to make a soft drink selection different from the rest of the school.
Cannot Measure Reliability
There is no way when using an opportunity sample to assess how representative the sample is of the population being studied. In the example of the English class, the students are all presumably of the same age group, influenced by the same English books and are subject to peer influence. It is impossible to measure if students of different ages, reading different English books or subject to other peer influences would respond differently. You simply do not know.