Margin of Error

Margin of Error deserves better than the throw-away line it gets in the bottom of stories about polling data. Writers who don't understand margin of error, and its importance in interpreting scientific research, can easily embarrass themselves and their news organizations.


Check out the following story that moved in the summer of 1996 on a major news wire

WASHINGTON (Reuter) - President Clinton, hit by bad publicity recently over FBI files and a derogatory book, has slipped against Bob Dole in a new poll released Monday but still maintains a 15 percentage point lead.


The CNN/USA Today/Gallup poll taken June 27-30 of 818 registered voters showed Clinton would beat his Republican challenger if the election were held now, 54 to 39 percent, with seven percent undecided. The poll had a margin of error of plus or minus four percentage points.


A similar poll June 18-19 had Clinton 57 to 38 percent over Dole.

Unfortunately for the readers of this story, it is wrong. There is no reasonable statistical basis for claiming that Clinton's lead over Dole has slipped.

Why? The margin of error. In this case, the CNN et al. poll had a four percent margin of error. 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. [emphasis added]