Overall error of the MCYFS

In this section a first look is given to the error made by MCYFS in forecasting EU-15 yields for the eight crops of interest. Averaged percentage error and absolute percentage error figures which have averaged years and months out are used. More specifically, the errors of each month and year are added, and the sum is divided by the number of errors in order to get the final figures. The period of reference is 1996-2002. This period has been chosen as it is the EU15 reference.

For each crop and month the Wilcoxon test is used to check whether the bias is significantly negative (underestimation) or positive (overestimation). The data for the test are annual averages for the given crop. The overall MPE and MAPE values of EU-15 yield forecasts for the period 1996-2002 (averages across years and months) are given in the table below.

Crop	MPE	P-VALUE	MAPE
Soft wheat	-0.38	0.87	3.48
Durum wheat	2.44	0.40	6.49
Barley	-1.21	0.50	2.71
Grain maize	-2.47		3.66
Rape seed	-0.22	1.00	5.03
Potato	-3.77	0.06	4.13
Sugar beet	-3.40	0.18	4.91
Sunflower	-3.24	0.12	5.54

These results show that MCYFS underestimates the yield of all crops except for Durum wheat. However, the apparent bias is not statistically significant, although in Potato this is marginally true. Moreover it can be observed that MAPE is beyond the 3% limit in all crops except Barley; for Durum wheat in fact it is beyond even the 6% limit.