Who Says 2+2=4?

What do Microsoft's Excel 2007 and I have in common? We're both math challenged, or so it seems.

In the case of Excel, Microsoft employee David Gainer reports in his blog that when users tried to get Excel 2007 to multiply some pairs of numbers and the result was 65,535, Excel would incorrectly display 100,000 as the answer. According to Gainer, Excel makes mistakes multiplying 77.1x850, 10.2x6425 and 20.4x3212.5. "Further testing showed a similar phenomenon with 65,536 as well," he said, adding that internally Excel calculates correctly but it is the display that messes stuff up. He went on to say that the problem is limited to six numbers from 65,534.99999999995 to 65,535, and six numbers from 65,535.99999999995 to 65,536.

In my case, I said in an editor's note entitled "2+2=What?" in today's Dr. Dobb's Report newsletter that "as when a 70% increase followed by a 60% decrease in statewide test scores is considered positive, even though the net effect on test scores is a 32% decrease." Well, if you actually compute that -- which I should have done before sending out the newsletter instead of after -- you get a 2% increase. As it turns out, the press release that I used as the source information had it backwards and, when I went back to the original document, it said "a 60% decrease followed by a 70% increase (resulting in a net decrease of 32%)". Thanks to readers Charles Linn and John Manis for quickly pointing this out.

Posted by Jon Erickson at 02:00 PM  Permalink