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- W2038581084 abstract "Abstract Conventional procedures for calculating confidence limits of forecasts generated by statistical models provide little guidance for forecasts based on a combination or a consensus process rather than formal models, as is the case with US Department of Agriculture (USDA) forecasts. This study applied and compared several procedures for calculating empirical confidence intervals for USDA forecasts of corn, soybean and wheat prices over the 1980/81 through 2006/07 marketing years. Alternative procedures were compared based on out-of-sample performance over 1995/96 through 2006/07. The results of this study demonstrate that kernel density, quantile distribution and best fitting parametric distribution (logistic) methods provided confidence intervals calibrated at the 80% level prior to harvest and 90% level after harvest. The kernel density-based method appears most accurate both before and after harvest with the final value falling inside the forecast interval 77% of the time before harvest and 92% after harvest, followed by quantile regression (73% and 91% before and after harvest, respectively) logistic distribution (73% and 90% before and after harvest, respectively) and histogram (66% and 84% before and after harvest, respectively). Overall, this study demonstrates that empirical approaches may be used to construct more accurate confidence intervals for USDA corn, soybean and wheat price forecasts. Acknowledgements The funding support of the USDA under cooperative Agreement 43-3AEK-5-80076 is gratefully acknowledged. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the USDA. Notes 1 It is worth noting that most theoretical variance expressions are based on the same assumption. 2 Isengildina et al. (Citation2004) provide survey evidence that USDA price intervals are symmetric. That is, a midpoint is forecast and then an equal interval is added to each side of the midpoint. Therefore, the average forecast price is computed in this study by taking an average of the midpoint of forecast prices for each month. 3 The last several months (17 and 18 for corn and soybeans and 14 and 15 for wheat) were not included in the analysis because errors were often zero, so the distributions were impossible to estimate. 4 It is important to note that the forecasting cycle for each of the commodities included in this analysis exceeds one calendar year. This implies that the final estimates (and thus forecast errors) for the previous year are not known until the 6th month into the next forecasting cycle for corn and soybeans and the 4th month for wheat. Therefore, the estimation sub-sample is one observation smaller in these early months of the forecasting cycle, i.e. while most intervals for 2006/07 are constructed using 26 prior observations, only 25 observations are available for the first 5 months of the corn and soybean forecasting cycle and the first 3 months of the wheat forecasting cycle. 5 This procedure was used by Isengildina et al. (Citation2004) to construct 95% confidence level forecast intervals and implied confidence level forecast intervals as described in footnote 9 of their paper. 6 The bin width of 0.05% was selected for this study, which was the finest bin based on the data, the coarser bin could have resulted in wider intervals. 7 Other distributions, such as gamma and Weibull were not included in this analysis because they are not defined for negative values. 8 To calculate the chi-squared statistic, one must break up the x-axis domain into ‘bins’ or classes of data. However, there are no clear guidelines for selecting the number and location of the bins and different conclusions can be reached from the same data depending on how bins are specified. 9 Computations were also made where the best-fitting distribution was allowed to change from year-to-year for corn. These alternative computations yielded quite similar results to those limited to the logistic distribution and are available from the authors upon request. 10 It is important to keep in mind that these interval widths are conditional on the selected confidence levels (80–90%). If the selected confidence level was equal to the actual hit rates, the widths of the empirical confidence intervals would be approximately equal to that of published intervals. 11 There is some evidence that mean forecast error of corn prices prior to harvest also shifted to the right, but the shift was not statistically significant. 12 Christoffersen (Citation1998) also proposed additional tests that examine interval forecast independence and forecast coverage conditional on independence. These tests, however, should only be applied across marketing years. For example, the May forecast for 2005/06 should be independent from the May forecast for 2006/07. Independence, however, is not possible between April and May forecasts for 2006/07, as these are forecasts of the same event. Therefore, our approach of combining pre- and post-harvest forecasts for accuracy analysis would yield meaningless results due to violation of independence across forecasts of the same events. Thus, due to a small number of observations, these tests cannot be applied reliably to the prediction part of the sample (1995/96–2006/07)." @default.
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- W2038581084 date "2011-10-01" @default.
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- W2038581084 title "Empirical confidence intervals for USDA commodity price forecasts" @default.
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