Year	per 1,000
1990	4.5
1991	4.3
1992	4.2
1993	4.1
1994	4
1995	3.9
1996	3.8
1997	3.7
1998	3.5
1999	3.4
2000	3.4
2001	3.3
2002	3.2
2003	3.1
2004	3
2005	2.9
2006	2.8
2007	2.7
2008	2.6
2009	2.5
2010	2.4
2011	2.3
2012	2.3
2013	2.2
2014	2.1
2015	2
2016	1.9
2017	1.8
2018	1.8
2019	1.7
2020	1.7
2021	1.6
2022	1.6