Year	% of total
1990	98.8
1992	98.6
1993	98.6
1994	98.5
1995	98.5
1996	98.4
1997	98.4
1998	98.3
1999	98.4
2000	99.9
2001	99.9
2002	99.9
2003	99.9
2004	99.8
2005	99.9
2006	99.8
2007	99.8
2008	99.8
2009	99.8
2010	99.8
2011	99.8
2012	99.8
2013	99.7
2014	99.7
2015	99.7
2016	99.6
2017	99.6
2018	99.6
2019	99.5
2020	99.6
2021	99.5