Year	per 1,000
1990	3.7
1991	3.6
1992	3.6
1993	3.5
1994	3.5
1995	3.4
1996	3.4
1997	3.3
1998	3.4
1999	3.4
2000	3.5
2001	3.7
2002	3.8
2003	4
2004	4.1
2005	4.1
2006	4
2007	3.9
2008	3.7
2009	3.5
2010	3.3
2011	3.1
2012	2.9
2013	2.7
2014	2.5
2015	2.3
2016	2.2
2017	2.1
2018	2
2019	2
2020	1.9
2021	1.9
2022	1.9