### Math 151 , Clarifying some similar concepts in sampling

Convenience Sample/ Voluntary Response Sample/ Sampling Frame/ Nonresponse bias/Response bias.
The edges between/among these concepts are a bit fuzzy.  Here's a try.
All of the following can lead to Undercoverage, a kind of selection bias: --the convenience sample and voluntary response sample almost for sure, the sampling frame possibly.  Some group(s) of the population are left out in choosing the sample..

Convenience Sample: You grab individuals or a group that is handy, and measure all of them. No attempt at a probability (random) sample.

Sampling frame:  A list of individuals that is not the whole population, but that you treat as the whole population. You draw a random sample from the sampling frame.  (Using the Campus Directory to sample students, or a Medical Society membership list to sample doctors, would be using a sampling frame.)  The sampling frame idea and the convenience sample overlap a little, since the sampling frame is probably some convenient list.  But you do try to get a sampling frame that is pretty representative of the population, and you  do try to get a probability sample from your sampling frame, even if the frame is not perfectly representative of the population.

Voluntary response sample:  You make a wide appeal for answers and let whoever wants to answer it (no attempt to get a probability sample).  Especially prone to Nonresponse bias.  Send-in, phone-in, magazine, website surveys are the paradigm.  Mail or email surveys to specific lists can slip into this category if there is no attempt to encourage response or to  follow up on the non-responders.

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No matter how careful you are of your sampling method, you can still incur Nonresponse bias if the people in your sample don't respond.
Nonresponse bias: Your sampled individuals don't all give you answers, and the individuals who don't answer are different from those who do (that's what makes for bias.)  Of course it's very hard to know if the individuals who don't answer are different, because they don't answer.  Nonresponse bias can happen even if you are careful about choosing the sample you ask.

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Response bias:  People answer but their answers are "off," conscious or unconscious lying.  Sometimes from wording or form of question, or even the order in which the questions are asked..

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Mail surveys that attempt to cope with the nonresponse problem use several strategies:
Paying you to respond--a dollar bill (or more!) in the envelope creates a sense of obligation; a gift certificate if you answer; entry into a sweepstakes if you answer; a promise of an analysis of the results of the survey if you answer.
Reinforcement--phoning you to urge you to send it in.
Coding the surveys so that they can track who responds, and sending second or even third and fourth attempts to the nonresponders. If they do this, they can compare the people who responded first to the people who responded on later appeals, and see if they are different.  They still don't know for sure about those who never respond, of course.

 Sievers home Math151-Sp12/Samplekinds.htm 3/6/12
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