| date | obsValue |
|---|---|
| 1990 | 274.2 |
| 1991 | 290.6 |
| 1992 | 281.6 |
| 1993 | 291.1 |
| 1994 | 280.6 |
| 1995 | 285.3 |
| 1996 | 310.0 |
| 1997 | 298.1 |
| 1998 | 298.3 |
| 1999 | 294.0 |
| 2000 | 290.9 |
| 2001 | 305.3 |
| 2002 | 298.6 |
| 2003 | 308.7 |
| 2004 | 308.7 |
| 2005 | 308.8 |
| 2006 | 305.5 |
| 2007 | 286.1 |
| 2008 | 300.3 |
| 2009 | 297.6 |
| 2010 | 317.0 |
| 2011 | 281.1 |
| 2012 | 295.4 |
| 2013 | 298.2 |
| 2014 | 263.2 |