Year	per 1,000
1990	3.8
1991	3.7
1992	3.6
1993	3.5
1994	3.4
1995	3.3
1996	3.2
1997	3.1
1998	3
1999	2.9
2000	2.8
2001	2.7
2002	2.6
2003	2.7
2004	2.4
2005	2.3
2006	2.2
2007	2.2
2008	2.1
2009	2.1
2010	2
2011	2
2012	2
2013	1.9
2014	1.9
2015	1.9
2016	1.8
2017	1.8
2018	1.7
2019	1.6
2020	1.5
2021	1.4
2022	1.4