Done - Edit 1
Before modification by Tor at 31/10/2012 09:34:49 AM
To produce a deviation as large as the one we are discussing here, a systematic bias is clearly needed.
No it isn't. You cannot expect a non-deviant result from a self-selected sample. Stop trying to insist otherwise. This is mathematical fact, not a matter of opinion.
So, I went and had a look at some statistics texts, and here are a few juicy quotes (emphasis mine):
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Self-selection bias is the problem that very often results when survey respondents are allowed to decide entirely for themselves whether or not they want to participate in a survey. To the extent that respondents' propensity for participating in the study is correlated with the substantive topic the researchers are trying to study, there will be self-selection bias in the resulting data.
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A voluntary response sample consists of people who choose themselves by responding to a general appeal. Voluntary response samples are biased because people with strong opinions, especially negative opinions, are most likely to respond.
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A sample of convenience is a sample that is not drawn by a well-defined random method. The big problem with samples of convenience is that they may differ systematically in some way from the population. For this reason, samples of convenience should not be used, except in situations where it is not feasible to draw a random sample. When it is necessary to draw a sample of convenience, it is important to think carefully about all the ways in which the sample might differ systematically from the population. If it is reasonable to believe that no important systematic difference exists, then it may be acceptable to treat the sample of convenience as if it were a simple random sample.
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Now, I've done as you asked, and in return, I would be very grateful if you would respond to my thought experiment from the other thread. I'm really quite pleased with it. I'll repeat it here, for you convenience:
So, thought experiment time:
You have a thousand people, all of which have a coin. You then ask anyone who feels like it to come forward, and flip their coin. I would expect roughly the same number of heads and tails, but would you? Remember, this is one of those scary self-selected samples.
Next, you have a thousand people, all of which have a coin. You tell them all to flip their coin, and use a magic marker to put a mark on the face that lands up, without looking at the result. You then ask whoever feels like it to come forward, and show you their coin. I would still expect roughly the same number of heads and tails, but would you?
Edit: Spelling.