Year	per 1,000
1990	3.7
1991	3.7
1992	3.6
1993	3.6
1994	3.6
1995	3.5
1996	3.5
1997	3.4
1998	3.4
1999	3.3
2000	3.3
2001	3.3
2002	3.2
2003	3.2
2004	3.1
2005	3.1
2006	3
2007	3
2008	2.9
2009	2.9
2010	2.8
2011	2.8
2012	2.7
2013	2.7
2014	2.7
2015	2.6
2016	2.6
2017	2.6
2018	2.6
2019	2.5
2020	2.5
2021	2.5
2022	2.5