By the numbers

Mathematical calculations sometimes carry a false aura of reliability; algorithm errors can lurk in mathematical programs. On Feb. 25, 1991, an error in Patriot missile weapons control software caused a failure that let a Scud missile hit an Army barracks, killing 28 Americans.

Investigators traced this to floating-point arithmetic calculations, which can generate wrong answers when they round off calculations for division. General Accounting Office investigators found that the system's registers were limited to 24 decimal places, letting the rounding error worsen progressively to the point that its radar failed to detect the Scud.

The Defense Advanced Research Projects Agency has been funding research to develop more accurate calculation tools. One approach relies on interval arithmetic, which assures that the correct answer to a problem lies within computed bounds.

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