| 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | |
|---|---|---|---|---|---|---|---|---|---|---|
| January | 6.63 | 6.66 | 6.55 | 6.24 | 5.87 | 5.20 | 5.03 | 5.22 | 5.00 | 4.84 |
| February | 6.59 | 6.75 | 6.37 | 6.14 | 5.86 | 5.04 | 5.11 | 5.18 | 4.45 | 4.84 |
| March | 6.71 | 6.72 | 6.45 | 6.15 | 5.69 | 5.22 | 5.23 | 5.17 | 4.74 | 4.84 |
| April | 6.63 | 6.68 | 6.39 | 5.98 | 5.71 | 5.17 | 5.10 | 5.14 | 4.70 | 4.84 |
| May | 6.74 | 7.00 | 6.52 | 5.94 | 5.75 | 5.26 | 5.21 | 4.91 | 4.63 | |
| June | 6.94 | 6.89 | 6.34 | 6.23 | 5.55 | 5.59 | 5.21 | 5.02 | 4.72 | |
| July | 7.08 | 6.76 | 6.01 | 6.23 | 5.41 | 5.51 | 5.43 | 5.07 | 5.11 | |
| August | 6.97 | 6.73 | 5.98 | 6.30 | 5.27 | 5.69 | 5.59 | 5.07 | 4.96 | |
| September | 7.01 | 6.70 | 6.35 | 6.29 | 5.31 | 5.46 | 5.52 | 5.18 | 5.10 | |
| October | 6.72 | 6.55 | 6.40 | 6.14 | 5.11 | 5.22 | 5.44 | 5.02 | 4.96 | |
| November | 6.86 | 6.38 | 6.19 | 6.02 | 5.21 | 5.08 | 5.50 | 5.14 | 4.87 | |
| December | 6.32 | 6.61 | 6.33 | 5.96 | 5.38 | 5.25 | 5.39 | 5.31 | 4.98 |