I'll admit it - election polling was a mystery to me. I hear one poll say Obama is ahead. Another says McCain. Another says they're in a dead heat.
How do we know which to believe? How do they determine who's ahead at any given moment?
So I decided to take a class, or rather a Webinar from Poynter/NewsU, called Understanding and Interpreting Polls in the 2008 Election. I highly recommend it. In the meantime, I'll share a few gold nuggets with you.
One of the first things you should know when trying to interpret a poll is how the participants were selected.
The best way to ensure an accurate result is to take a random sample from a specific population. If it's not random, it's not mathematically accurate. The polling industry standard is the RDD, or random digit dial. This is a poll taken by telephone where numbers are not taken from a list, but instead are dialed randomly. Since a large number of the population has telephones, it excludes few enough people that it can still be statistically accurate.
Since women are statistically more likely to answer the phone, sometimes you will have someone ask for the man of the house, or even ask for someone born in a particular month. This is the pollster's effort to even out the possible bias if mostly women were to participate because they simply answered the phone. So they're not just being sexist!
Another method for selecting poll participants is registration sampling. This is when the list is derived from registered voters. This can give a good picture, but may exclude the all-important swing voters or new voters.
Both of these methods are called probability polling, because they draw random samples from defined populations.
Another alternative for selecting poll participants is non-probability sampling. This includes self-selected poll participants. For example, if you're watching Lou Dobbs on CNN, he sometimes has call-in polls. While this can give a reliable sample of Lou Dobbs watchers, it does not give an accurate measure of the whole country. Someone who is watching Lou Dobbs, who cares enough to pick up the phone and self-selects to participate is not representative of the voters who do not watch CNN or who instead are watching Jerry Springer or All My Children.
Be wary of these TV call-in polls (on any channel) for predicting the mood of the country - they are not statistically reliable.
What about Internet polls? While they are fast, easy, and private, pollsters do not yet know how to get a random sample of Internet addresses. They cannot make them up, like they can with phone numbers (imagine all the crazy e-mail addresses you know). And there is no consolidated directory of all Internet addresses. Another downfall of Internet polls - they do not know who is actually participating - it could even be a kid! Again, Internet polls usually involve self-selection, which are not probability polls and do not give accurate measures.
When you're looking at poll results, they usually have a little box somewhere on the page that shows the Methodology. Take a look to see if you can tell how the poll participants were selected, and take that into account when assessing the credibility of the poll numbers you are reading.
Next we'll explore other important factor in understanding polls, like timing and state polls v. national polls. Stay tuned!
Source: Poynter/NewsU, Claudia Dean: Understanding and Interpreting Polls in the 2008 Election, http://www.newsu.org/courses/course_list.aspx (accessed 9/18/08)