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Da Vinci math and other tales of innumeracy

Referencing “The Da Vinci Code” in the title ought to guarantee some readership.
And it is partly accurate, just like several of the “facts” examined here.

Referencing “The Da Vinci Code” in the title ought to guarantee some readership.

And it is partly accurate, just like several of the “facts” examined here.

We love to believe we traffic in facts, and few things look more factlike than numbers. However, by failing to look at the right number, in the right way, we are often misled. Three recent examples caught my attention.

“The Da Vinci Code” represents a failure to think numerically. I was not one of the 90 million people who bought the book and had managed to keep the plot a surprise when I walked into the movie. Emerging from the theater, I heard others debating historicity and theology, but my mind went to math.

Taking the premise as a given — that Jesus and Mary Magdalene had a child 2,000 years ago — the chances that there would be just one surviving descendant is near zero. In fact, it is vastly more likely that every human being alive today is a descendant of that union than that there is only one person in the “blood line.”

Simplify the problem greatly and witness the power of exponential growth. Let’s say the daughter allegedly produced by Jesus and Mary had two children, and in each of the 85 succeeding generations (2,000 years divided by 25-year generations) each of those progeny produced two children. Two to the 85th power is — well let’s just say it’s over a quadrillion times larger than the current population of the planet.

Of course, that is vastly oversimplified. But sophisticated mathematical models and computer simulations reveal that about 60 percent of those who were alive 2,000 years ago are the ancestors of every individual alive today, while the remainder have no descendants. In short, Jesus and Mary could have either billions of descendants or none, but not just one.

Newsweek offers a second example of misusing numbers. Twenty years late, it corrected its oft quoted quip that a single 40-year-old woman is “more likely to be killed by a terrorist” than to marry.

Not only was the basic arithmetic wrong, but the calculations were plagued by a series of conceptual problems as well. One was the assumption that nothing changes — that women born in the ’60s would display the same pattern of behavior as those born in the ’40s.

Newsweek’s demographers simply turned the facts about one era into predictions about another. They assumed that if few women born in the 1940s married after age 40, the results would be the same for women born in the 1960s. In fact, the median age for marriage reached its 20th century low in 1956. Failing to account for changing tastes or altered environments dooms many attempts to extrapolate the past into the present and future.

Finally, consider money madman Jim Cramer’s contention on the April 30 “Meet the Press” that soaring gas prices would not affect consumers. “The consumer’s shockingly resilient,” he declared. “Yesterday, Wal-Mart [reported a] … 6.8 percent increase in comparable [April] store sales over last year. Remarkable. ... It shows me the consumer … is not being impacted” by gas prices. That is until the next month, when Wal-Mart’s CEO blamed sluggish sales on — you guessed it — high gas prices.

“Wal-Mart Stores did miss their plan,” the executive moaned, “as our customer continues to be impacted by higher gas prices.”

Cramer missed the fact that this year’s April sales were stronger than last year’s because, in 2006, Easter fell in April rather than in March. Moreover, gasoline prices did not hit $3 a gallon until late that month.

Facts are helpful tools, but only if one know how to use them.

Mellman is president of The Mellman Group and has worked for Democratic candidates and causes since 1982, including Sen. John Kerry (D-Mass.) in 2004.