Year	per 1,000
1990	3.4
1991	3.3
1992	3.2
1993	3.2
1994	3.1
1995	3.1
1996	3.1
1997	3.1
1998	3.1
1999	3.1
2000	3
2001	3
2002	2.9
2003	2.9
2004	2.8
2005	2.7
2006	2.6
2007	2.5
2008	2.3
2009	2.2
2010	2.1
2011	2
2012	2
2013	1.9
2014	1.8
2015	1.8
2016	1.7
2017	1.6
2018	1.6
2019	1.5
2020	1.4
2021	1.3
2022	1.3