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The grocery market in the United States accounts for more than 25 percent of all retail sales, totaling hundreds of billions of dollars. The last decade has seen important transformations in the market structure of this industry with non-traditional retailers such as Walmart and Costco accounting for more than 30 percent of total grocery sales. For any given product, the spread of consumer prices from different retailers is orders of magnitude larger than price fluctuations from changing world food market conditions. The average consumer faces prices that differ by more than 20 percent across stores in the same area.