Hi I missed this when it was first asked but I just read the two (I think that's the number) replies to the list and both seem well informed to me. I wish I could write as much and as well about statistics even though I understand both replies. =;+) I will add this. That also 900 or so units of analysis should provide a good normal distribution. I use magic numbers for samples like this 12, 20, 30, 50, 100, 300. 12 is enough for small sample T test and as you go to larger samples you get closer to a normal distribution and more tests become justified. Also Fred's reply got me thinking that you could check with linear regression diagnostics ( See a book on this. Neuter et al Linear Models is my bible) to see if the needed assumptions (Fred listed these) hold for linear regression. Also I have studied in drug law about survey's of large populations like returning Vietnam veterans and national health survey's so I do not believe that you have it right about there being no survey's with probabilistic sampling. May be I am wrong but you should check in the drug law literature to verify your statement. What is interesting in these survey's is that we find most addicts recover and become drug free, yet if you ask hospitals they do not say that most addicts recover. This is also the same for mental illness. I think it has to do with the hospitals only seeing the problem cases thus as observers they are biased. I would also think those who use the Internet and openly talk about illegal drugs online are different from those addicts who know they will be monitored by posting online and do not post. This suggests if true that like Fred was saying your sample would have bias and you would have to qualify your sample with some statement about the units of analysis opening up online. The population at large may not open up online. Also in self help groups confidentiality is practiced so nothing would tend to show up online about these meetings. Peter president of an unmentioned self help group at present. On 6-Sep-09, at 7:32 AM, Monica Barratt wrote:

Thanks to everyone who has responded, publicly and privately. I'll get back to this thread over the next week, as I expect to get further input from a few other contacts soon too.

Monica _______________________________________________ The Air-L@listserv.aoir.org mailing list is provided by the Association of Internet Researchers http://aoir.org Subscribe, change options or unsubscribe at: http://listserv.aoir.org/listinfo.cgi/air-l-aoir.org

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Peter Timusk statistical computer programmer ptimusk@sympatico.ca address 701-151 Parkdale Avenue Ottawa, Ontario Canada K1Y 4V8 Phone 613-729-8328 May all your numbers be quality numbers... even if they are only average numbers.