Year	per 1,000
1990	3.6
1991	3.4
1992	3.3
1993	3.2
1994	3.1
1995	3
1996	2.9
1997	2.8
1998	2.7
1999	2.6
2000	2.5
2001	2.4
2002	2.3
2003	2.3
2004	2.2
2005	2.1
2006	2.1
2007	2
2008	1.9
2009	1.9
2010	1.8
2011	1.7
2012	1.7
2013	1.6
2014	1.6
2015	1.5
2016	1.5
2017	1.4
2018	1.4
2019	1.3
2020	1.3
2021	1.3
2022	1.2