Super Tuesday Preview: Don’t believe the hype, remember the margin of error

Tomorrow, 22 states vote in presidential primaries. As nearly half the country prepares to vote, BlogHer will be on hand with dispatches and video blogging from our contributors’ states. You’re our eyes and ears on the ground, so please comment on our Open Thread tomorrow! For news, delegate information, and candidate schedules, see The New York Times' Election Guide. For the best rundown of polling updates and state by state info, Professor Kim sent us a link to Joy Reid's blog. There is just one word of caution I want to impart, courtesy of my fantastic statistics teacher Deb Hughes Hallett. The horse race coverage is a dangerous thing. You’ll hear that Hillary Clinton is down to a “single digit lead” in many key states, with Obama closing in. When you read exit polls Tuesday, you’ll hear that Hillary may be winning, say, 38% to 34%, or McCain is beating Romney by 3% points. Remember, in 2004 midday exit polls said Kerry was beating Bush, 50% to 48%....and we know what happened.

A single digit lead means it’s too close to call, because of the margin of error. The margin of error goes like this (I found a great example online, at Robert Niles’ blog, which I have adapted for this race). Say you see a headline (fictional numbers here) that says Obama has closed a lead over Clinton and now Clinton leads by 8% in New Jersey, whereas two weeks ago she led by 11%. Say the poll has a 4% margin of error, which would be reasonable for a small sample. “That means that if you asked a question from this poll 100 times, 95 of those times the percentage of people giving a particular answer would be within 4 points of the percentage who gave that same answer in this poll.” It doesn't mean Obama gained 3 points.

There is no reasonable statistical basis for claiming that Clinton's lead over Obama has slipped.


Want to know more about the margin of error? From Deb:

A poll of 250 votes gives the largest margin of error, about 6.2%. 
For a poll of 450 voters, the margin of error is about 4.6%.



What to do with the margin of error:
 If the difference between the two candidate’s polls is less than the margin of error, you should say the race is too close to call. (Reason: The difference you are seeing could easily be the result of sampling variation in the poll.)


If the difference is more than twice the margin of error, you can say that the person who is ahead is likely to win. (In this case the difference is large enough that it is unlikely to be the result of sampling variation in the poll.)


What if the poll results are between one and two margins of error apart? This is the grey area, where it is possible that the candidate with lower poll numbers will actually win. The chance of this goes down as the difference between them gets closer to two margins of error. 

Let's assume that 450 people were polled: The margin of error is 4.6%. 
If there is a 9% difference between candidates, the top candidate probably will win.
 If the difference between candidates is less than 4.5%, the race is too close to call.
 Between 4.5% and 9%, you could to say something like “X appears to be ahead at the moment, but keep watching the polls.”





Look out for the spin tomorrow. I guarantee you will read and hear journalists proclaiming victory when things are well within the margin of error!

So you are you voting for? Still undecided? Here are endorsements and considerations from bloggers I respect:

Obama, NY: Over at Culture Kitchen, Liza Sabater boasts an Obama splash page and writes, “I believe the measure of a leader is in their ability to inspire their supporters creatively.” Ps, if you haven’t see the “Yes We Can” video. Liza has it here http://culturekitchen.com/liza/blog/video_will_i_ams_remix_of_barack_oba...

Faye Anderson covers Obama and Clinton, strengths and weaknesses

Conservative Media Lizzy: McCain is hard for Bush girls to accept!

A community of largely Edwards supporters,Chicago Moms have a good interview with Clinton surrogate Judith Lichtman.

Mary Katharine Ham: I complained a lot about McCain tonight, and didn't mean to jump on the super-anti-McCain bandwagon. I have serious problems with him, but I'm more inclined to support him than probably a lot of conservative bloggers are, since I can see accepting his expertise on the war and his ability to beat Hillary or Obama as a trade-off given the alternative. Maybe. But such is the magic of John McCain that it's as if he went into the debate looking to tick people off.

McCain is a bitter pill for many Republicans to swallow. But seemingly, an inevitable one. St. Louis radio host Dana Loesch "can't stomach McCain as the knighted leader of American conservatism."

Simply Kimberly rounds up bloggers for Mitt, though, and they still have Hope:

I will admit that a large part of my support for Mitt Romney comes from being so vehemently anti-McCain. Others may call this McCain Derangement Syndrome. But it really stems from knowing McCain's record and not believing that he would make any better President of the United States than he has made a United States Senator. The definition of insanity is doing the same thing over and over and expecting different results. (Ben Franklin).

Most Democrats I know are thrilled with their candidates. It is thrilling.

See you Tuesday!