| Period
| 2006 | 2007 | 2008 | 2009 | 2010 | | January | 11 | 10 | 10 | 13.55 | 13.5 | | February | 11 | 11 | 10 | 14 | 13.29 | | March | 11 | 11 | 10 | 14 | | | January-March | 11 | 10.7 | 10 | 13.84 | | | April | 11 | 11 | 10 | 14 | | | May | 11 | 11 | 10 | 14 | | | June | 10.5 | 11 | 10 | 14 | | | April-June | 10.8 | 11 | 10 | 14 | | | January-June | 10.9 | 10.8 | 10 | 13.92 | | | July | 10.5 | 10.75 | 10.25 | 14 | | | August | 10.5 | 10.5 | 10.4 | 14 | | | September | 10.5 | 10.25 | 10.5 | 14 | | | July-September | 10.5 | 10.5 | 10.4 | 14 | | | January-September | 10.8 | 10.72 | 10.1 | 13.95 | | | October | 10.5 | 10 | 10.64 | 14 | | | November | 10.5 | 10 | 10.91 | 14 | | | December | 10 | 10 | 11.48 | 13.5 | | | October-December | 10.3 | 10 | 11.01 | 13.83 | | | January-December | 10.7 | 10.5 | 10.35 | 13.92 |
| Notes: * – calculated based on the calendar number of days.
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