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Wednesday, January 19, 2005

Sample selection bias?

One of the hardest lessons for econometrics students is the concept of sample selection and its potential to bias results.

Case in point: exit polling during the Nov. 2004 election. More Democrat voters took the exit poll than Republican voters and therefore the exit polls suggested the Kerry was going to win. A new report suggests that the exit polling was flawed and there doesn't seem to be any evidence of systematic vote fraud:
Exit polls do not support the allegations of fraud due to rigging of voting equipment. Our analysis of the difference between the vote count and the exit poll at each polling location in our sample has found no systematic differences for precincts using touch screen and optical scan voting equipment," the report found.

At least they are getting around to recognizing that there is a problem:

The new report shows that exit polls overstated Kerry's support in 26 states, while estimates overstated Bush's support in four states. The problem is not new -- in every presidential election since 1988, exit polls have overstated support for Democrats nationally -- but the discrepancy in 2004 was more pronounced than in previous years.

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