Key months for collecting data were selected on data published by the CFMU for the ECAC area and available at Eurocontrol. A qualitative assessment was done based on the daily traffic recorded by the CFMU and shown in Figure 26.
Figure 26: Average monthly traffic in the CFMU area from 1997 to 2001. (Source: CFMU, 2002).
The analysis of Figure 26 shows that the year can be divided into four parts according to the seasons. During the winter period, a constant increase of the traffic can be noticed from January to March. At spring, the increase started in winter carries on from April to June with a slightly steeper slope. In summer, only small variations are observed between July and August while September appears to be the busiest month of the year. In autumn a steady decrease is observed from October to December. This analysis led to the selection of the following months for collecting data:
February, April, June, September, October and December.

February and April were chosen as intervals of time representing the increase observed during winter and spring. June and September announce the beginning and end of the summer season with a higher number of flights than the annual average for 2001 (Table 40). The events of the 11th of September modified the market for the end of the year. However, based on the five-year trend, October initiates the decrease of the number of flights and December the end with the least flights in the year.
Table 40: Yearly traffic averages for the ECAC area from 1997 to 2001 (Source: CFMU, 2002).
1997  1998  1999  2000  2001  Average 1997-2001 
Daily traffic in the ECAC area (number of flights)  19662  20685  22062  23068  22995  21694 

To ensure the six months retained were properly chosen, the proportion of annual traffic represented by these six months was calculated. The analysis showed that these six months accounted for roughly 49% of 2000’s and 2001’s traffic in the ECAC area (Table 41).
Table 41: Monthly traffic in 2000 and 2001 for the ECAC area (Source: CFMU, 2002).
Month  Monthly traffic in 2000  Monthly traffic 2000  in  Monthly traffic in 2001  Monthly traffic in 2001 
Number of flights  %  Number of flights  % 
January  623804  7.39  654635  7.80 
February  623275  7.38  612330  7.30 
March  690681  8.18  699459  8.33 
April  676452  8.01  691183  8.24 
May  751662  8.90  751421  8.95 
June  743599  8.81  753254  8.97 
July  761903  9.02  772110  9.20 
August  766299  9.08  776145  9.25 
September  761251  9.02  753600  8.98 
October  746574  8.84  732617  8.73 
November  672274  7.96  627108  7.47 
December  625007  7.40  569356  6.78 

Total  8442781  100  8393218  100 
6 months selected  49  49 

2.1.1. Selection of the key week in the ECAC area
The evolution of the traffic for all weeks between 1999 and 2001 is shown in Figure 27. From the graphs, no trend could be drawn from one year to the other. A strong variation can occur from week to week within a month. Therefore, the selection of a period within the selected month was done arbitrarily. The second week of the month was chosen for February (week 6), April (week 15), June (week 24), September (week 37) and October (week 41). The first week of December (week 49) was retained for deadline project reason and the period of Christmas, which would cause delay in the collection of the data.
If comparing the choice of the week for collecting data in 2002 and the weekly traffic in 2001 for the full months selected (Figure 28), it can be noticed that:
　
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