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