year yield  effort
1990 159.28 310.087
1991 144.81 301.492
1992 138.16 314.541
1993 128.43 288.132
1994 131.38 304.220
1995 145.62 254.281
1996 139.68 255.804
1997 131.26 291.778
1998 163.76 281.334
1999 150.87 270.000
2000 180.36 259.842
2001 159.97 277.777
2002 145.51 304.640
2003 159.87 254.598
2004 161.16 214.738
2005 134.30 150.019