Year	per 1,000
1990	3.9
1991	3.8
1992	3.8
1993	3.7
1994	3.7
1995	3.6
1996	3.5
1997	3.5
1998	3.4
1999	3.3
2000	3.3
2001	3.2
2002	4.8
2003	3.1
2004	3.1
2005	3.1
2006	3.1
2007	3.1
2008	3.1
2009	3.1
2010	3
2011	3
2012	2.9
2013	2.9
2014	2.8
2015	2.7
2016	2.7
2017	2.6
2018	2.6
2019	2.5
2020	2.4
2021	2.4
2022	2.3