Look at this graph:
It's a simulation of what statistical sampling error alone would do to the percentage margin between Obama and McCain in the Diageo/Hotline/National Journal/Atlantic Monthly daily tracking poll if the race were tied and if there was no motion at all in underlying voter preferences.
The point has been made before by Matt Yglesias:
The Three Day Itch: One thing a week’s vacation from blogging helps you get perspective on is the Gallup tracking poll. On August 1 when I had my last day at The Atlantic it was time for panic.... Then Obama started to regain ground, going up to a four point lead.... In short, McCain’s “Celebrity” ad and drilling attacks were working well, but when the McCain campaign went after Obama on the tire gauge thing he came up with effective countermeasures and regained his lead. Maybe. Or maybe none of that happened. As everyone knows, there’s sampling error associated with polling....
Alan Abramowitz points out if you look at the daily results... you see... incredible volatility.... [If they reported the daily samples] people could see that this is basically statistical noise in a stable race. But Gallup doesn’t report its daily results, they report a... three day rolling average, [and] you get these pleasant looking peaks and valleys in the race. The change over time here is large enough in magnitude... but also slow enough in pace (unlike on the one day chart) to be plausibly interpreted as public opinion shifting in response to events. And since the human mind is designed to recognize patterns and construct narratives, and since it suits the interests of campaign journalists to write narratives, people interpret the peaks and valleys of the three-day average as real shifts in public opinion...
And people are wrong to so interpret the peaks and valleys of the three-day average.
Are there reasons to trust any of their publications? To pay them any money? To buy things made by their advertisers (including Diageo, the world's leading beer, spirits and wine company)? To employ their pollsters who actually carry out the survey (Financial Dynamics, with Democrat Ed Reilly and Republican Brent McGoldrick as principles).
At least, I can't think of any reason. Maybe you can.
Why oh why can't we have a better press corps?