Concentration and Specialization in United States Agriculture, 1939-1978
利用1939-1978年美国19种农产品的销售数据，通过区位商和差异指数分析，发现多数农产品的销售变得更加集中和区域专业化，且其分布与人口分布的相似性降低。
It has been hypothesized that reduced transportation costs will result in increasing concentration and regional specialization in agriculture. Yet, modern agricultural innovation would appear to reduce comparative advantage of agricultural production throughout a nation. Using United States farm sales data for 19 agricultural commodities gathered between 1939 and 1978, it was ascertained through use of the location quotient and index of dissimilarity that the farm sales of the majority of commodities had become both more concentrated and regionally specialized. Furthermore, the distribution of farm sales of most commodities has grown less similar to that of the nation's population during the same period.