Year	per 1,000
1990	4.2
1991	4.1
1992	3.9
1993	3.7
1994	3.6
1995	3.4
1996	3.3
1997	3.2
1998	3.1
1999	3
2000	2.9
2001	2.8
2002	2.7
2003	2.6
2004	2.5
2005	2.5
2006	2.4
2007	2.3
2008	2.3
2009	2.2
2010	2.2
2011	2.1
2012	2
2013	2
2014	1.9
2015	1.9
2016	1.8
2017	1.8
2018	1.7
2019	1.7
2020	1.6
2021	1.6
2022	1.5