Year	per 1,000
1990	4.4
1991	4.2
1992	4.1
1993	3.9
1994	3.8
1995	3.7
1996	3.6
1997	3.5
1998	3.4
1999	3.3
2000	3.3
2001	3.2
2002	3.1
2003	3.1
2004	3
2005	2.9
2006	2.8
2007	2.8
2008	2.7
2009	2.6
2010	2.5
2011	2.5
2012	2.4
2013	2.3
2014	2.3
2015	2.2
2016	2.2
2017	2.1
2018	2
2019	2
2020	1.9
2021	1.9
2022	1.8