Year	% of total
1987	100
1991	99.1
1992	99.6
1993	99.4
1994	99.3
1995	99.4
1996	99.2
1997	99.3
1998	99.3
1999	99.2
2000	99.3
2001	99.3
2002	99.3
2003	98.9
2004	99
2005	98.8
2006	100
2007	98.8
2008	98.7
2009	98.6
2010	98.5
2011	98.4
2012	98.1
2013	98
2014	97.9
2015	97.9
2016	97.8
2017	97.9
2018	98
2019	98
2020	98