The Numbers Guy examines numbers in the news, business and politics. Some numbers are flat-out wrong or biased, while others are valid and help us make informed decisions. Carl Bialik tells the stories behind the stats, in daily updates on this blog and in his column published every other Friday in The Wall Street Journal. Carl, who holds a degree in mathematics and physics from Yale University, also cowrites The Daily Fix, a sports column on WSJ.com. He welcomes your comments at .

WSJ.com: The Numbers Guy
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Carl Bialik examines the way numbers are used, and abused.

A Lifetime of Career Changes
Though its often said that the average American will have seven careers in a lifetime, there is no statistical support for that figure.

Keeping Tabs on Traffic
How traffic engineers are seeking to use data and mathematical models to ease congestion.

Putting Teachers to the Test
Value-added measures have many problems, but some researchers and advocates say these teacher-evaluation tools based on student test scores, may be better than the alternative.

Elusive Numbers: U.S. Population by Religion
The Census Bureau doesn't ask about religion, unlike other countries' censuses, opening the door to controversies about the number of Muslims and other U.S. religious groups.

Tracking the Image Trackers
How polling firms and other companies track corporate reputation, from monitoring social networks to surveying thousands of people daily.

The Census’s 21st-Century Challenges
Participation in the U.S. census is mandatory, but the law is rarely enforced. Should the country go to a registration system for cheaper, more reliable stats?

The World’s Best Agers
How demographers are beginning to uncover the statistics of the world's oldest people.

Studying a Suicide Cluster
To analyze whether a recent spate of suicides at a set of Chinese manufacturing facilities represents an unusual outbreak, it helps to make the right comparisons.

The Case of Apple and the Mysterious Bars
Apple's announcement that its formula for calculating signal bars to display on the iPhone was "totally wrong" spotlighted a mysterious aspect of cellphone technology.

When Polling Numbers Don’t Look Random
In an unusually public rift, a prominent left-wing political Web site is renouncing polling it had commissioned and published and is suing its former pollster.