Year	per 1,000
1990	3.4
1991	3.4
1992	3.4
1993	3.3
1994	3.3
1995	3.2
1996	3.2
1997	3.1
1998	3
1999	2.9
2000	2.8
2001	2.7
2002	2.6
2003	2.5
2004	2.4
2005	2.3
2006	2.2
2007	2.1
2008	2
2009	1.9
2010	1.8
2011	1.7
2012	1.6
2013	1.5
2014	1.5
2015	1.4
2016	1.4
2017	1.3
2018	1.3
2019	1.2
2020	1.1
2021	1.1
2022	1