Year	Percent
1991	18.061
1992	17.571
1993	17.434
1994	17.983
1995	18.121
1996	17.443
1997	17.165
1998	16.358
1999	16.857
2000	16.059
2001	16.284
2002	16.509
2003	17.068
2004	17.604
2005	17.487
2006	17.657
2007	17.829
2008	17.839
2009	18.095
2010	18.415
2011	18.553
2012	19.051
2013	18.917
2014	19.494
2015	19.075
2016	19.503
2017	19.396
2018	19.337
2019	19.27
2020	19.169
2021	19.054
2022	18.941