| Year | March | September | March trend | September trend |
|---|---|---|---|---|
| 1979 | 16.608 | 7.474 | ||
| 1980 | 16.323 | 8.082 | ||
| 1981 | 15.87 | 7.734 | ||
| 1982 | 16.411 | 7.748 | ||
| 1983 | 16.313 | 7.903 | ||
| 1984 | 15.883 | 7.375 | ||
| 1985 | 16.2 | 7.244 | ||
| 1986 | 16.177 | 7.858 | ||
| 1987 | 15.986 | 7.692 | ||
| 1988 | 16.226 | 7.694 | ||
| 1989 | 15.646 | 7.277 | ||
| 1990 | 16.081 | 6.497 | ||
| 1991 | 15.557 | 6.799 | ||
| 1992 | 15.637 | 7.752 | ||
| 1993 | 15.922 | 6.753 | ||
| 1994 | 15.75 | 7.513 | ||
| 1995 | 15.326 | 6.444 | ||
| 1996 | 15.325 | 7.88 | ||
| 1997 | 15.683 | 6.959 | ||
| 1998 | 15.753 | 6.908 | ||
| 1999 | 15.489 | 6.501 | ||
| 2000 | 15.326 | 6.667 | ||
| 2001 | 15.648 | 7.059 | ||
| 2002 | 15.448 | 6.164 | ||
| 2003 | 15.779 | 6.494 | ||
| 2004 | 15.135 | 6.235 | ||
| 2005 | 14.818 | 5.761 | ||
| 2006 | 14.577 | 6.279 | ||
| 2007 | 14.772 | 4.628 | ||
| 2008 | 15.409 | 5.223 | ||
| 2009 | 15.306 | 5.759 | ||
| 2010 | 15.395 | 5.395 | ||
| 2011 | 14.815 | 5.05 | ||
| 2012 | 15.548 | 4.055 | ||
| 2013 | 15.151 | 5.543 | ||
| 2014 | 15.08 | 5.679 | ||
| 2015 | 14.598 | 5.143 | ||
| 2016 | 14.648 | 5.024 | ||
| 2017 | 14.539 | 5.251 | ||
| 2018 | 14.523 | 5.215 | ||
| 2019 | 14.784 | 4.761 |