| Year | January | February | March | September | Mar trend | Sep trend |
|---|---|---|---|---|---|---|
| 1978 | 16.59 | 8.95 | ||||
| 1979 | 15.84 | 16.57 | 16.85 | 8.0 | 16.54 | 8.85 |
| 1980 | 15.59 | 16.46 | 16.51 | 8.75 | 16.5 | 8.76 |
| 1981 | 15.43 | 16.08 | 16.14 | 8.17 | 16.46 | 8.66 |
| 1982 | 15.72 | 16.46 | 16.53 | 8.25 | 16.41 | 8.57 |
| 1983 | 15.45 | 16.42 | 16.53 | 8.45 | 16.37 | 8.47 |
| 1984 | 14.91 | 15.67 | 15.98 | 7.88 | 16.33 | 8.38 |
| 1985 | 15.11 | 15.83 | 16.32 | 7.8 | 16.29 | 8.28 |
| 1986 | 15.27 | 16.21 | 16.3 | 8.15 | 16.24 | 8.19 |
| 1987 | 15.51 | 16.45 | 16.17 | 8.15 | 16.2 | 8.09 |
| 1988 | 15.58 | 16.0 | 16.35 | 8.16 | 16.16 | 8.0 |
| 1989 | 15.22 | 15.89 | 15.86 | 7.75 | 16.11 | 7.9 |
| 1990 | 15.14 | 16.0 | 16.34 | 6.94 | 16.07 | 7.81 |
| 1991 | 14.79 | 15.65 | 15.8 | 7.24 | 16.03 | 7.71 |
| 1992 | 15.3 | 15.97 | 16.04 | 8.24 | 15.98 | 7.62 |
| 1993 | 15.46 | 16.14 | 16.22 | 7.32 | 15.94 | 7.52 |
| 1994 | 15.19 | 16.02 | 15.97 | 8.02 | 15.9 | 7.43 |
| 1995 | 15.06 | 15.64 | 15.66 | 6.82 | 15.85 | 7.33 |
| 1996 | 14.61 | 15.55 | 15.53 | 8.42 | 15.81 | 7.24 |
| 1997 | 14.97 | 15.86 | 15.9 | 7.42 | 15.77 | 7.14 |
| 1998 | 15.12 | 16.1 | 16.02 | 7.5 | 15.73 | 7.05 |
| 1999 | 14.84 | 15.66 | 15.71 | 7.18 | 15.68 | 6.95 |
| 2000 | 14.72 | 15.61 | 15.64 | 7.28 | 15.64 | 6.86 |
| 2001 | 14.76 | 15.68 | 15.85 | 7.59 | 15.6 | 6.76 |
| 2002 | 14.89 | 15.77 | 15.75 | 6.59 | 15.55 | 6.67 |
| 2003 | 14.78 | 15.55 | 15.84 | 6.89 | 15.51 | 6.57 |
| 2004 | 14.43 | 15.27 | 15.31 | 6.64 | 15.47 | 6.48 |
| 2005 | 14.04 | 14.77 | 15.04 | 6.19 | 15.42 | 6.38 |
| 2006 | 13.91 | 14.67 | 14.77 | 6.56 | 15.38 | 6.29 |
| 2007 | 14.14 | 14.91 | 14.91 | 4.96 | 15.34 | 6.19 |
| 2008 | 14.24 | 15.27 | 15.44 | 5.39 | 15.29 | 6.09 |
| 2009 | 14.22 | 15.06 | 15.26 | 5.89 | 15.25 | 6.0 |
| 2010 | 13.97 | 14.88 | 15.41 | 5.55 | 15.21 | 5.9 |
| 2011 | 13.79 | 14.63 | 14.87 | 5.12 | 15.16 | 5.81 |
| 2012 | 14.04 | 14.9 | 15.47 | 4.15 | 15.12 | 5.71 |
| 2013 | 0.0 | 0.0 | 15.43 | 6.16 | 15.08 | 5.62 |