A self-selected sample that is randomly selected from a population has no specification of sampling error--as the term is meaningless in that context. A more technical online introduction with a bit of math from National Science Foundation is SESTAT's Understanding Sampling Errors and What is the Margin of Error. News Release of January 29, 2004 - "Maryland Security Study Validates Diebold Election Systems Equipment for March Primary." URL:
Their practices and results are of doubtful value, to say the least. New Telemarketing Ploy Steers Voters on Republican Path, New York Times, 12/6/06. Odum Institute Poll Item Database Query Page has properly worded questions ° Answers to Questions We Often Hear from the Public from National Council on Public Polls ° If You're Going to Poll by The Why Files; see its Polling Glossary (for layman's explanation of standard terminology in polls), Serious Statistical Secrets page (good explanation of the basics), Obey the Law (law of large numbers, that is), Doing it wrong... (on subtle failures associated with not taking a truly random sample), Oops!! on deliberative polling per James Fishkin of U of Texas (my alma mater) and his Goes a Long Way and Why Change (of opinions) on the National Issues Convention in Austin, TX. Students often mistakenly identify polls on controversial subjects to be ugly polls. It is perfectly legitimate for good polls to address the most touchy or delicate subjects. In fact, those are often the things most worthwhile to know and understand.