Year	Number
1987	22.48785
1988	20.5436
1989	19.73488
1990	18.10136
1991	18.44831
1992	20.35838
1993	22.49556
1994	23.63777
1995	24.97859
1996	25.04354
1998	27.18801
1999	28.24302
2000	26.32665
2001	26.33391
2002	26.24333
2003	26.07569
2004	28.44722
2005	30.98364
2006	26.19698
2014	33.0932
2015	33.54677
2016	26.36771
2017	23.70599
2018	25.9167