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