Active Users:364 Time:17/06/2025 07:34:06 AM
Done Tor Send a noteboard - 31/10/2012 09:34:11 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.
Fram kamerater!
This message last edited by Tor on 31/10/2012 at 09:34:49 AM
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The Bell Curve revisited - 29/10/2012 09:44:09 AM 1526 Views
Re: The Bell Curve revisited - 29/10/2012 10:21:27 AM 910 Views
That's incorrect... - 29/10/2012 10:26:49 AM 1483 Views
Re: That's incorrect... - 29/10/2012 10:36:32 AM 912 Views
RJ the physicist didn't know math, so that Shannow could be right... - 29/10/2012 02:11:19 PM 828 Views
Response to a few of your poorly researched points... - 29/10/2012 02:31:17 PM 772 Views
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Exactly... - 29/10/2012 02:39:30 PM 787 Views
there are dozens of reasons for this - 29/10/2012 08:18:18 PM 805 Views
Excellent point. - 29/10/2012 08:24:37 PM 834 Views
Re: there are dozens of reasons for this - 29/10/2012 09:07:35 PM 736 Views
Again I don't argue that genetics play no role - 30/10/2012 01:57:24 AM 713 Views
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And why would it be biased towards those with lower strength? - 29/10/2012 09:11:25 PM 696 Views
Absolutely no reason... - 30/10/2012 01:35:35 AM 794 Views
Re: Absolutely no reason... - 30/10/2012 06:43:54 AM 697 Views
Only if it was a random sampling. Which this is not. - 30/10/2012 01:58:34 PM 784 Views
That's exactly the point. I want you to explain why it wasn't random. - 30/10/2012 02:14:59 PM 707 Views
It wasn't random because it was a self-selected sample! - 30/10/2012 02:43:03 PM 722 Views
Re: It wasn't random because it was a self-selected sample! - 30/10/2012 02:47:30 PM 720 Views
Go read a stats text will you? - 30/10/2012 02:54:16 PM 715 Views
Done - 31/10/2012 09:34:11 AM 1452 Views
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Re: You seem to have perfected whining to a Talent... - 11/11/2012 11:37:16 AM 812 Views
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Still nothing? - 10/11/2012 03:33:15 PM 798 Views
Still doesn't explain the difference - 30/10/2012 07:01:53 PM 663 Views
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Yes that totally makes sense - 30/10/2012 08:07:16 AM 834 Views
Thank you! *NM* - 30/10/2012 10:19:15 AM 386 Views
That's not what happened... - 30/10/2012 02:01:52 PM 762 Views
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Who said it would? - 30/10/2012 02:44:17 PM 721 Views
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But of course he did.. - 30/10/2012 02:13:07 PM 822 Views
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Overwhelm Lanfear, not match her. *NM* - 29/10/2012 08:26:09 PM 421 Views
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Its highly specific... - 30/10/2012 02:15:38 PM 661 Views
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Re: In the truest sense, you are probably right that it is skewed - 29/10/2012 09:10:27 PM 763 Views
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Couldn't the Towers method of obtaining Aes Sedai be to blame? - 30/10/2012 12:04:01 AM 904 Views
Re: Couldn't the Towers method of obtaining Aes Sedai be to blame? - 30/10/2012 09:33:44 AM 829 Views
Are you sure about that? - 30/10/2012 12:03:43 PM 830 Views
Re: Are you sure about that? - 30/10/2012 12:19:34 PM 736 Views
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Re: Sharina did not have the Spark, nor did Nicola - 30/10/2012 05:54:41 PM 725 Views
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+1 *NM* - 30/10/2012 09:17:07 AM 833 Views
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Not true... - 30/10/2012 11:49:57 AM 816 Views
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