| March | June | September | December | |
|---|---|---|---|---|
| 2025 | 4.48 | 4.40 | 4.34 | 4.20 |
| 2024 | 4.30 | 4.55 | 4.71 | 4.62 |
| 2023 | 2.74 | 3.15* | 3.59 | 4.00 |
| 2022 | 0.32 | 0.75 | 1.35 | 2.07 |
| 2021 | 0.44 | 0.33 | 0.24 | 0.23 |
| 2020 | 2.03 | 1.47* | 0.84 | 0.63 |
| 2019 | 2.55 | 2.57 | 2.45 | 2.29 |
| 2018 | 1.51 | 1.90 | 2.16 | 2.40 |
| 2017 | 0.78 | 0.92 | 1.07 | 1.20 |
| 2016 | 0.46 | 0.55 | 0.60 | 0.68 |
| 2015 | 0.26 | 0.28 | 0.32 | 0.37 |
| 2014 | 0.23 | 0.22 | 0.24 | 0.25 |
| 2013 | 0.28 | 0.24 | 0.26 | 0.26 |
| 2012 | 0.38 | 0.36 | 0.35 | 0.32 |
| 2011 | 0.51 | 0.48 | 0.38 | 0.38 |
| 2010 | 0.56 | 0.56 | 0.51 | 0.46 |
| 2009 | 1.91 | 1.51 | 0.90 | 0.60 |
| 2008 | 4.18 | 3.11 | 2.77 | 2.54 |
| 2007 | 5.17 | 5.23 | 5.24 | 4.96 |
| 2006 | 4.03 | 4.53 | 4.93 | 5.11 |
| 2005 | 2.38 | 2.85 | 3.18 | 3.63 |
| 2004 | 1.47 | 1.44 | 1.67 | 2.00 |
| 2003 | 1.98 | 1.77 | 1.63 | 1.56 |
| 2002 | 2.96 | 2.75 | 2.63 | 2.31 |
| 2001 | 6.16 | 5.32 | 4.47 | 3.52 |
| 2000 | 5.80 | 6.18 | 6.47 | 6.52 |
| 1999 | 5.19 | 5.08 | 5.21 | 5.49 |
| 1998 | 5.70 | 5.66 | 5.64 | 5.46 |
| 1997 | 5.56 | 5.63 | 5.68 | 5.71 |
| 1996 | 5.62 | 5.52 | 5.57 | 5.58 |
| 1995 | 5.76 | 5.98 | 5.89 | 5.76 |
| 1994 | 4.25 | 4.45 | 4.96 | 5.37 |
| 1993 | 4.64 | 4.51 | 4.44 | 4.36 |
| 1992 | 5.87 | 5.45 | 4.97 | 4.67 |
| 1991 | 7.97 | 7.38 | 7.00 | 6.52 |
| 1990 | 8.52 | 8.50 | 8.39 | 8.27 |
| 1989 | 8.76 | 9.13 | 8.87 | 8.68 |
| 1988 | 8.01 | 7.87 | 8.20 | 8.45 |
| 1987 | 7.24 | 7.21 | 7.54 | 7.97 |
| 1986 | 9.09 | 8.39 | 7.81 | 7.48 |
| 1985 | 10.32 | 9.98 | 9.54 | 9.43 |
| 1984 | 10.32 | 10.88 | 11.53 | 11.41 |
| 1983 | 9.87 | 9.64 | 10.04 | 10.18 |
| 1982 | 11.82 | 11.99 | 11.74 | 10.71 |
| 1981 | 11.23 | 11.68 | 12.40 | 11.91 |
| 1980 | 11.11 | 11.54 | 10.01 | 10.47 |
| 1979 | 8.81 | 9.10 | 9.26 | 10.06 |
| 1978 | 6.97 | 7.35 | 7.86 | 8.32 |
| 1977 | 5.68 | 5.78 | 5.84 | 6.45 |
*Revised