Voluntary Response Sample — Definition, Formula & Examples
A voluntary response sample is a sample in which participants choose themselves whether to respond, rather than being selected by the researcher. Because only people with strong opinions tend to respond, the results are almost always biased.
A voluntary response sample is a nonprobability sampling method in which individuals self-select into the sample, typically in response to an open invitation such as a survey, online poll, or call-in vote. This method systematically overrepresents individuals with strong feelings about the topic, producing response bias that prevents valid generalization to the population.
Example
Problem: A TV news station asks viewers to call in and vote on whether the city should raise property taxes. Of the 4,800 callers, 87% say "no." Can the station conclude that 87% of all city residents oppose the tax increase?
Identify the sampling method: Viewers chose whether to call in. No one was randomly selected, so this is a voluntary response sample.
Identify the likely bias: People who feel strongly against the tax are far more likely to take the time to call. Residents who are indifferent or mildly in favor are underrepresented.
Draw a conclusion: The 87% figure is not trustworthy as an estimate for the entire city. The voluntary response design overrepresents opponents of the tax.
Answer: No. The voluntary response sample is biased toward people with strong negative opinions, so the station cannot generalize the 87% result to all city residents.
Why It Matters
On the AP Statistics exam, identifying a flawed sampling method and explaining the direction of bias is a common free-response task. Voluntary response samples appear frequently as the textbook example of bias caused by self-selection. Recognizing this flaw is also practical — online polls, product reviews, and social media surveys all suffer from it.
Common Mistakes
Mistake: Confusing voluntary response sampling with convenience sampling.
Correction: In a voluntary response sample, the respondent decides to participate. In a convenience sample, the researcher chooses whoever is easiest to reach. Both are biased, but the source of bias differs: self-selection vs. researcher accessibility.