| Year | March | September | March trend | September trend |
|---|---|---|---|---|
| 1979 | 16.846 | 8.003 | ||
| 1980 | 16.512 | 8.751 | ||
| 1981 | 16.14 | 8.173 | ||
| 1982 | 16.531 | 8.22 | ||
| 1983 | 16.525 | 8.455 | ||
| 1984 | 15.981 | 7.88 | ||
| 1985 | 16.315 | 7.799 | ||
| 1986 | 16.298 | 8.145 | ||
| 1987 | 16.174 | 8.146 | ||
| 1988 | 16.349 | 8.156 | ||
| 1989 | 15.856 | 7.749 | ||
| 1990 | 16.336 | 6.941 | ||
| 1991 | 15.797 | 7.241 | ||
| 1992 | 16.038 | 8.24 | ||
| 1993 | 16.222 | 7.323 | ||
| 1994 | 15.969 | 8.023 | ||
| 1995 | 15.655 | 6.817 | ||
| 1996 | 15.531 | 8.417 | ||
| 1997 | 15.897 | 7.416 | ||
| 1998 | 16.023 | 7.497 | ||
| 1999 | 15.71 | 7.184 | ||
| 2000 | 15.638 | 7.281 | ||
| 2001 | 15.848 | 7.593 | ||
| 2002 | 15.753 | 6.595 | ||
| 2003 | 15.841 | 6.894 | ||
| 2004 | 15.31 | 6.636 | ||
| 2005 | 15.036 | 6.187 | ||
| 2006 | 14.767 | 6.563 | ||
| 2007 | 14.913 | 4.962 | ||
| 2008 | 15.436 | 5.39 | ||
| 2009 | 15.258 | 5.893 | ||
| 2010 | 15.367 | 5.597 | ||
| 2011 | 14.826 | 5.23 | ||
| 2012 | 15.448 | 4.224 | ||
| 2013 | 15.27 | 5.86 | ||
| 2014 | 15.118 | 5.945 | ||
| 2015 | 14.709 | 5.486 | ||
| 2016 | 14.877 | 5.394 | ||
| 2017 | 14.711 | 5.545 | ||
| 2018 | 14.675 | 5.744 |