# year    value 
 1980 17.62629 
 1981 17.53003 
 1982 17.42117 
 1983 17.04072 
 1984 17.22214 
 1985 17.39934 
 1986 17.43253 
 1987 17.55036 
 1988 17.56855 
 1989 17.70027 
 1990 17.75623 
 1991 17.68582 
 1992 17.30779 
 1993 17.57039 
 1994 17.58204 
 1995 17.57772 
 1996 17.78357 
 1997 18.00176 
 1998 17.90386 
 1999 17.95108 
 2000 18.10461 
 2001 18.20576 
 2002 18.05721 
 2003 18.05083 
 2004 18.06428 
 2005 18.30079