and give weekly and monthly average prices in the major markets for long periods going back to the 1960s and earlier. An alternative would be to use statistics on farm harvest prices, but these are usually averages with unknown weights of different varieties, the coverage is uneven and uncertain, and it is difficult to obtain reasonably lengthy time series. Experiments using farm harvest prices were tried in the detailed studies of coarse grains and of pulses, and in an adjunct study of wheat and rice, but were judged to be unsatisfactory.

A fixed proportion ( and in some cases 100 percent) of some commodities has been acquired by the government at "levy prices " which are lower than open market wholesale prices. In these cases we have calculated and used the weighted average of the levy price and the free market wholesale price in estimating nominal protection, where the weights are the mandated share of levy sales in production and the free market share. This is important for sugar, for which the government has varied the levy proportion of mill sales quite substantially from year to year, and also for molasses where the entire production of each sugar mill has been acquired at very low levy prices. 2 The domestic prices used in the estimation of nominal protection are intended to approximate as closely as possible the prices that the farmers (or mills in the case of estimates done at that level) receive during the harvest. Prices during the rest of the year reflect a variety of other factors including storage costs of traders, and using them to measure incentives would include the incentives for post harvest storage and distribution functions as well as farming. The principle we have followed has therefore been to calculate average prices during the periods during which the bulk of each crop is harvested. These periods are indicated in the individual studies. But it should be noted that they differ considerably as between commodities e.g. most of the wheat crop in the north-west is harvested and sold during a short six week period, whereas rubber and coconut trees in the south produce year round. There are also differences for the same commodities between producing regions. In addition the data available for some commodities(e.g. rubber) in earlier years is for "crop year " averages, which differ from Indian fiscal years. Some of the harvest periods also overlap fiscal years but have been assigned to particular fiscal years in presenting the estimates. The correspondences between these periods and fiscal years are indicated in the individual studies, but they should borne in mind in interpreting the time series, and especially the NPC estimates for agriculture as a whole and the various other aggregations.

There can be substantial differences between the prices of different varieties and grades of a commodity, and we have been careful to specify which ones have been used in the price comparisons. As far as possible we have used price data for the same or similar varieties in the estimates for each year, including the long term estimates. But doing so has inevitably involved making some _ad hoc_ judgments which are noted in the text or the relevant tables, and also in the original commodity studies. The varieties for which price comparisons have been made are generally those with the largest production that the authors of the original commodity studies 2 If the levy were of a fixed quantity rather than a fixed proportion, it could be argued that the free market price should be used, as this is the relevant incentive that producers will take into account in deciding whether to increase or decrease their production. On the other hand in the long run the size of the levy and the acquisition price will affect the total return from the crop and long run investment and production decisions. In practice the Indian levies have been applied to processors (mainly sugar mills and rice mills)and have been fixed proportions of mill production. There has been no corresponding differentiation of prices paid to farmers: the main effect of the levies has been to reduce the prices that mills can afford to pay for sugar cane and paddy respectively.

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