| Year | Standard Deviation (SD) | Range (R) |
| 1995 | 0.010 | 0.04 |
| 1996 | 0.010 | 0.04 |
| 1997 | 0.011 | 0.05 |
| 1998 | 0.013 | 0.05 |
| 1999 | 0.014 | 0.06 |
| 2000 | 0.014 | 0.06 |
| 2001 | 0.015 | 0.07 |
| 2002 | 0.016 | 0.07 |
| 2003 | 0.017 | 0.08 |
| 2004 | 0.018 | 0.09 |
| 2005 | 0.020 | 0.10 |
| 2006 | 0.019 | 0.09 |
| 2007 | 0.019 | 0.09 |
| 2008 | 0.020 | 0.10 |
| 2009 | 0.019 | 0.10 |
| 2010 | 0.020 | 0.10 |
| 2011 | 0.021 | 0.10 |
| 2012 | 0.023 | 0.12 |
| 2013 | 0.024 | 0.13 |
| 2014 | 0.024 | 0.12 |
| 2015 | 0.025 | 0.13 |
| 2016 | 0.025 | 0.13 |
| 2017 | 0.026 | 0.14 |
| 2018 | 0.026 | 0.14 |
| 2019 | 0.027 | 0.15 |