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