Year	per 1,000
1990	3.5
1991	3.2
1992	2.9
1993	2.8
1994	2.6
1995	2.6
1996	2.5
1997	2.5
1998	2.4
1999	2.4
2000	2.4
2001	2.4
2002	2.4
2003	2.4
2004	2.3
2005	2.2
2006	2
2007	1.9
2008	1.8
2009	1.8
2010	1.7
2011	1.6
2012	1.5
2013	1.5
2014	1.4
2015	1.4
2016	1.3
2017	1.3
2018	1.3
2019	1.3
2020	1.3
2021	1.3
2022	1.3