Estimating the level of protection: The implications of seasonal price fluctuations
研究发现，用年度平均价格估算名义保护系数等指标会因季节性价格波动产生偏差，因为价格与交易量负相关，未加权平均价格高估了贸易加权平均价格，并提出了估计贸易权重的经验方法。
The Nominal Protection Coefficients (NPCs), Effective Protection Coefficients (EPCs), and Producers Subsidy Equivalents (PSEs) are usually estimated from annual prices which are the unweighted average of seasonal prices. In the presence of seasonal fluctuations in prices, the NPCs, EPCs, and PSEs, estimated from annual data, can seriously over/underestimate the actual protection received by the producers and consumers. The bias results from the fact that price and quantity traded normally relate negatively, and unweighted average price consistently overestimates the trade-weighted average price. The bias in the NPC estimates is demonstrated both theoretically and empirically. In most cases, quantity traded at each price is not known, and the paper suggests empirical methods for estimating the trade weights.