LESSON: Sampling Methods

In a voluntary sample, subjects "self-select" for participating in the survey. They may be interested in or feel strongly about the topic, they may want the perks/prizes for participating or have any other number of motivations for choosing to participate.

Television and radio stations often ask viewers/listeners to call in with opinions about a particular issue they are covering. The websites for these and other organizations also usually include some sort of online poll question of the day. Reality television shows and fan balloting in professional sports to choose all-star players make use of these types of polls as well. All of these polls usually come with a disclaimer stating that, “This is not a scientific poll.” 

While perhaps entertaining, these types of polls are very susceptible to voluntary response bias. The people who respond to these types of surveys tend to feel very strongly one way or another about the issue in question, and the results might not reflect the overall population. Those who still have an opinion, but may not feel quite so passionately about the issue, may not be motivated to respond to the poll. This is especially true for phone-in or mail-in surveys in which there is a cost to participate. The effort or cost required tends to weed out much of the population in favor of those who hold extremely polarized views. 

A news channel might show a report about a child killed in a drive-by shooting and then ask for people to call in and answer a question about tougher criminal sentencing laws. They would most likely receive responses from people who were very moved by the emotional nature of the story and wanted anything to be done to improve the situation. 

An even bigger problem is present in those types of polls in which there is no control over how many times an individual may respond.

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