Order picking and material handling, in general, have received considerable attention since the 1970's. It is estimated that the overall logistics cost in the United States is 21% of the Gross National Product, and that 28% of this logistics cost is accounted for by order pick systems (OPSs). There are more than 300,000 large warehouses with 2.5 million employees in the United States, and the cost of warehousing is more than 5% of the Gross National Product. Some 63% of all operating costs in a typical warehouse can be attributed to order picking. According to a survey of the Warehouse Education and Research Council, order picking has been identified as the number one area for improvement in warehousing.