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When their own money's at stake . . . . . . American shoppers become walking computers. Grocery store shoppers make on-the-spot, price-comparison calculations with 98 percent accuracy, according to a five-year study on decisionmaking by a University of California anthropologist. Yet the same shoppers do much more poorly on conventional, sit-down math tests of roughly the same difficulty.

Further, notes Jean Lave, a UC Irvine anthropologist: ''Not one of (the shoppers tested) recognized that she never made a mistake in doing supermarket math.''

Education level and the years that had passed since school were closely tied to how well the shoppers did on math tests. But in the store, there was no connection between schooling and figuring accuracym.