Year | Value | 1999 | 13.597441809398 | 1998 | 14.347658670665 | 1997 | 11.420361726954 | 1996 | 13.438996579247 | 1995 | 16.678206578459 | 1994 | 15.616054772724 | 1993 | 14.928916309224 | 1992 | 15.122783962781 | 1991 | 15.142662979358 | 1981 | 11.609348019015 | 1980 | 13.005071555075 | 1979 | 14.094486735478 | 1978 | 14.965322476389 | 1977 | 13.478576733728 | 1976 | 11.368925110406 | 1975 | 8.8559906131326 | 1974 | 10.676915721142 | 1973 | 10.346908860978 |
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