| Year | Min Rate | Max Rate |
| 1971 | 7.29 | 7.73 |
| 1972 | 7.23 | 7.46 |
| 1973 | 7.43 | 8.85 |
| 1974 | 8.40 | 10.03 |
| 1975 | 8.80 | 9.60 |
| 1976 | 8.70 | 9.10 |
| 1977 | 8.65 | 9.00 |
| 1978 | 8.98 | 10.38 |
| 1979 | 10.38 | 12.90 |
| 1980 | 12.18 | 16.35 |
| 1981 | 14.80 | 18.63 |
| 1982 | 13.57 | 17.66 |
| 1983 | 12.55 | 13.89 |
| 1984 | 13.14 | 14.68 |
| 1985 | 11.09 | 13.29 |
| 1986 | 9.29 | 10.99 |
| 1987 | 9.03 | 11.58 |
| 1988 | 9.84 | 10.77 |
| 1989 | 9.68 | 11.22 |
| 1990 | 9.56 | 10.67 |
| 1991 | 8.35 | 9.75 |
| 1992 | 7.84 | 9.03 |
| 1993 | 6.74 | 8.07 |
| 1994 | 6.97 | 9.25 |
| 1995 | 7.11 | 9.22 |
| 1996 | 6.94 | 8.42 |
| Year | Min Rate | Max Rate |
| 1997 | 6.99 | 8.18 |
| 1998 | 6.49 | 7.22 |
| 1999 | 6.74 | 8.15 |
| 2000 | 7.13 | 8.64 |
| 2001 | 6.45 | 7.24 |
| 2002 | 5.93 | 7.18 |
| 2003 | 5.21 | 6.44 |
| 2004 | 5.38 | 6.34 |
| 2005 | 5.53 | 6.37 |
| 2006 | 6.10 | 6.80 |
| 2007 | 5.96 | 6.74 |
| 2008 | 5.10 | 6.63 |
| 2009 | 4.71 | 5.59 |
| 2010 | 4.17 | 5.21 |
| 2011 | 3.91 | 5.05 |
| 2012 | 3.31 | 4.08 |
| 2013 | 3.34 | 4.58 |
| 2014 | 3.80 | 4.53 |
| 2015 | 3.59 | 4.09 |
| 2016 | 3.41 | 4.32 |
| 2017 | 3.78 | 4.30 |
| 2018 | 3.95 | 4.94 |
| 2019 | 3.49 | 4.51 |
| 2020 | 2.66 | 3.72 |
| 2021 | 2.65 | 3.18 |
| 2022 | 3.22 | 7.08 |
| 2023 | 6.09 | 7.23 |