Sampling -- Voluntary Sample
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"Voluntary Samples" are both the worst kind of sample and one of the most common kind
of sample. The essence of a voluntary sample is that we do not force items to be in
the sample. Certainly, this is generally related to getting samples of people.
[We would not have to ask an "M&M" if it
wants to be in a sample of "M&M's" where we are looking at the
color distribution of those candies!]
You have almost certainly been approached to be such
a voluntary sample.
You get phone calls asking if you will participate in a study by answering a few
questions, you get emails asking you to give feedback on items,
you are asked to fill out response
cards at restaurants and other stores, and so forth.
An even more pervasive
example of such voluntary sampling is web sites that give you a place to
give ratings or reviews.
In each case you get to choose
if you do or do not want to participate.
Such sampling does not represent
the general population because it ignores all of the people who do not want to
respond to the survey.
Furthermore, surveys that are open for people to
decide to participate attract participants who tend to have strong feelings about
the particular issue.
It is easy to create a web site that asks people to
answer a few questions about how they view a certain topic.
Those people who have
strong feelings about that topic will not only vote (and vote often) they will get their like-minded
friends to search out and join the survey.
However, the possible vast majority of
people who have feelings and opinions on the subject but who may not be so rabid will just
not participate.
As a consequence, any report of the "result" of the
survey cannot be assumed to represent anyone but the
people who actually responded to the survey.
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©Roger M. Palay
Saline, MI 48176 December, 2015