There is no cut-and-dried answer on what makes a poll 'valid.' All public reports of survey findings should include reference to the following:

• Sponsorship of the survey
• Dates of interviewing
• Method of obtaining the interviews (in-person, telephone or mail)
• Population that was sampled
• Size of the sample
• Size and description of the sub-sample, if the survey report relies primarily on less than the total sample
• Complete wording of questions upon which the release is based
• The percentages upon which conclusions are based

Key things to look for are sample size and corresponding margin of error (be cautious of results when the margin of error exceeds 5 or 6 points); whether the sample was scientifically chosen or whether respondents were self-selected (typical of '900' call-in polls and many web-site polls); and the wording and order of the questions (look for questions that seem to 'push' the respondent to an answer by describing some options more attractively than others).

More information is available in the short publication Twenty Questions A Journalist Should Ask About Poll Results by Sheldon Gawiser and G. Evans Witt. The pamphlet is available from the National Council on Public Polls.