Year	% of total
1990	100
1991	99.9
1992	99.8
1993	99.8
1994	99.8
1995	99.8
1996	99.8
1997	99.8
1998	99.9
1999	99.6
2000	99.3
2001	99.5
2002	99.4
2003	99.4
2004	99.4
2005	99
2006	98.9
2007	98.9
2008	98.7
2009	98.8
2010	98.2
2011	97.9
2012	97.8
2013	98.1
2014	97.8
2015	98.2
2016	97.9
2017	98
2018	98.2
2019	98.3
2020	97.4