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