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