Year	per 1,000
1990	4.3
1991	4.4
1992	4.4
1993	4.4
1994	4.3
1995	4.1
1996	3.9
1997	3.7
1998	3.6
1999	3.5
2000	3.4
2001	3.3
2002	3.3
2003	3.2
2004	3.2
2005	3.1
2006	3
2007	2.9
2008	2.8
2009	2.6
2010	2.5
2011	2.3
2012	2.2
2013	2
2014	1.9
2015	1.9
2016	1.8
2017	1.8
2018	1.8
2019	1.8
2020	1.8
2021	1.9
2022	1.9