RELATIVE VARIABILITY IN WHEAT YIELDS ACROSS COUNTRIES AND OVER TIME
分析了57个国家1951-1986年小麦产量的变异系数，发现国家规模、水分和温度是主要影响因素，而技术变量无显著影响；发展中国家自1975年后产量变异性普遍下降，印度地区数据也证实了这一点。
Wheat yield variability is analysed in light of recent concern that rapid technological change has caused increased instability in world cereal production. The coefficient of variation of wheat yields is estimated for 57 countries from detrended data for various periods between 1951 and 1986. The coefficient of variation in wheat yields is shown to be determined by country size, moisture regime and temperature. Technological variables, such as level of adoption of high‐yielding varieties and fertiliser dose, had no effect on differences in yield variability across countries. Analysis of yield variability for the same set of countries for three periods from 1951 to 1986 shows a general decline in yield variability since 1975 in developing countries. Analysis of wheat yield variability in India at the state and district levels confirms the analysis of country level data. The coefficient of variability of wheat yields in India in the period 1976‐85 has fallen to less than half the level in the 1950s and this decline is statistically significant.