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A voluntary response sample is a made up of individuals who volunteer to be included in the sample.
For example, suppose a radio host asks listeners to go online and take a survey on his website about their opinion of his show. Each individual listener can voluntarily decide to take the survey or not.
The drawback of this sampling method is that the individuals who voluntarily respond will likely have stronger opinions (positive or negative) than the rest of the population, which makes them an .
The visual below illustrates this problem: suppose the green circles represent people who think highly of the radio show while the red circles represent people who dislike the show:
Notice how most of the people who think highly of the show are included in the sample, yet the sample is not representative of the larger population. The results of the survey would show that most people like the show, when in fact this is not true.
Voluntary response sampling is a type of because not every individual in the population has an equal probability of being included in the sample.
This is in contrast to probability-sampling methods, in which each individual in the population is equally likely to be included in the sample and thus the sample is likely to be representative of the overall population.
Examples of Voluntary Response Samples
The following scenarios illustrate a couple more examples of voluntary response samples.
Example 1: Exam Prep
Suppose a professor wants to know if a new exam prep course helps students improve test scores. She posts a sign-up sheet outside of her classroom and lets students decide if they’d like to participate in the course.
This is an example of voluntary response sampling because students can voluntarily decide to be included in the sample.
Unfortunately, students who are more studious are more likely to sign up which means the sample of students who take the course aren’t likely to match the overall population of students who could potentially take the course.
Example 2: Traffic Law
Suppose researchers want to know what citizens in a particular city think of a new traffic law so they mail out a questionnaire to every citizen.
This is an example of voluntary response sampling because each individual citizen can decide to be included in the sample or not.
Disadvantages of Voluntary Response Samples
Voluntary response sampling suffers from the following types of bias:
1. : When some members of a population are inadequately represented in the sample.
2. : When individuals select themselves to be included in a survey.
3. : When the people who respond to a survey differ significantly from the people who do not respond to the survey.
Due to all of these types of bias, voluntary response samples end up not being representative of the overall population of interest.
Despite so many drawbacks, voluntary response sampling is often used simply because it’s an easy way to collect a sample since individuals voluntarily decide to include themselves.