Don't trust that tool.

WHAT WOULD REMBRANDT'S PAINTING "The Night Watch" look like if his brushes kept breaking in mid-stroke?

The question is analogous to a problem engineers face when using spreadsheet programs not explicitly designed for engineering calculations, according to Alan Stevens, an engineer

Tools that aren't designed for engineering calculations can introduce errors, which the computer interface often disguises, Stevens said. Engineers don't readily identify the errors.

In 1973, Stevens joined Rolls-Royce's submarine business, where he performed thermal and hydraulic calculations for the design, safety, and performance of the nuclear reactor cores that drive Royal Navy submarines. He's now a specialist in mathematical modeling and simulation, with particular interest in how general-purpose mathematical tools are used by engineers, he said.

Stevens said that he has documented several instances in which the popular spreadsheet program Microsoft Excel returns wrong answers. In one experiment, he enlisted several Rolls-Royce engineers to run calculations. He found that Excel didn't accurately calculate the location of the center of a circle and its radius because it failed to precondition the data, a problem that worsens with the increasing size of a data set, Stevens said.

"Talented engineers are using Excel and getting serious errors, of which they're simply not aware," Stevens said. "And errors build up more rapidly than you might expect."

Stevens uses Mathcad software from Mathsoft Engineering & Education Inc. of Cambridge, Mass., for many of his own engineering calculations, he said.