I admit it... polls affect my mood when it comes to politics.  When my candidate comes off well in the polls, I get excited and happy.  When he comes off poorly, I get depressed.  But my friend James has cautioned me more than once, not to let polls affect my mood this way.  You really can't trust them, he says...

First of all, people who answer polls are a special category of people.  Often these are people who have some sort of vested interest in affecting the outcome of the poll.  If you're a die hard supporter of candidate X (who isn't doing well in the polls) and you are asked to take a poll, you're probably going to take the poll so as to improve your candidate's standing and give him momentum.  Needless to say this slants the poll.

An undecided voter is probably less likely to take a poll, but more likely to review the results of one.

Secondly, the people administering the poll unintentionally introduce bias in the manner that they seek applicants.  An online poll, for example, is accessible to people who are computer literate and have access to computers.  There are surely a large number of potential voters who don't fall into that group, and further, their politcal views may differ considerably.  Someone with no computer access is more likely to be poorer for example.  Poorer people tend to have different political concerns than richer people.

Thirdly, polls are often manipulated by unscrupulous people. They can be affected by the people who create the poll by creating leading questions like "How important is Bush's committment to protecting you from terrorists to you?" followed by "How important is Kerry's inconsistent flip/flopping to you?"  They can be affected by folks who lie in their answers on the poll to affect the outcome, such as for example, Republican politcal consultant pretending to be an "undecided voter" so he can affect opinion.

Fourthly, people tend to ignore the margin of error on polls.  "Bush has a 3% lead with a margin of error of 3%."  Which means basically, according to this poll Bush could be leading by as much as 6% or as little as 0%!  Further it implies an accuracy that might not be there.  Remember polls can be manipulated.  Can you really trust that the pollsters who report a 10% lead with a 2% margin of error don't have some sort of agenda?

Fifthly, there are literally dozens of polls available, and taken together they often don't agree.

Finally, neither polls nor pollsters know who is going to vote.  A candidate may make a suicidal flub on the day before voting.  Unforseable events (attacks, sickness, weather catastrophes, voting problems, etc.) may affect voter turnout, and in unpredictable areas.  Any of these things affect turnout, and here's the scary part.  Polls themselves, affect turnout!!!

Over on MotherJones.com Michael Schwartz writes an excellent critique of polls and polling (The Opiate of the Electorate) in which he urges the reader not to lend much credence to polls, no matter what they say.  He summarizes with the following cautionary note:

Three things are worth remembering, if you can't kick the poll-watching habit:
  1. Any individual poll can be off by 15%.
  2. Any collection of honestly conducted polls, looked at together, will show a very wide range of results and you won't be able to tell which of them is right.
  3. Even the collective results of a large number of polls probably will not give you an accurate read on a close election.
From these three points comes the most important conclusion of all -- don't let the polls determine what you think or what you do.

I highly recommend this article, if the polls have been getting you down like they get me down.