Year	per 1,000
1990	4.5
1991	4.5
1992	4.4
1993	4.4
1994	4.3
1995	4.2
1996	4.1
1997	4
1998	4
1999	4
2000	3.9
2001	3.8
2002	3.6
2003	3.4
2004	3.1
2005	3
2006	2.9
2007	2.8
2008	2.7
2009	2.6
2010	2.5
2011	2.4
2012	2.3
2013	2.2
2014	2.1
2015	2
2016	2
2017	1.9
2018	1.8
2019	1.8
2020	1.7
2021	1.7
2022	1.7