Year	per 1,000
1990	3.3
1991	3.2
1992	3.1
1993	3
1994	3
1995	3
1996	3.1
1997	3.2
1998	3.3
1999	3.5
2000	3.6
2001	3.6
2002	3.5
2003	3.4
2004	3.3
2005	3.1
2006	3
2007	2.9
2008	2.8
2009	2.7
2010	2.6
2011	2.5
2012	2.3
2013	2.1
2014	2
2015	1.9
2016	1.9
2017	1.9
2018	1.9
2019	1.9
2020	1.8
2021	1.8
2022	1.8