Year	per 1,000
1990	4.8
1991	4.7
1992	4.6
1993	4.5
1994	4.5
1995	4.4
1996	4.3
1997	4.3
1998	4.2
1999	4.1
2000	4.1
2001	4
2002	4
2003	3.9
2004	3.9
2005	3.8
2006	3.7
2007	3.6
2008	3.5
2009	3.4
2010	3.3
2011	3.2
2012	3.1
2013	3.1
2014	3
2015	3
2016	2.9
2017	2.9
2018	2.9
2019	2.8
2020	2.8
2021	2.7
2022	2.7