"Consumers are generally more sensitive to changes in prices than to changes in quantity," John T. Gourville, a marketing professor at Harvard Business School, said. "And companies try to do it in such a way that you don't notice, maybe keeping the height and width the same, but changing the depth so the silhouette of the package on the shelf looks the same. Or sometimes they add more air to the chips bag or a scoop in the bottom of the peanut butter jar so it looks the same size."

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