Year	per 1,000
1990	3.5
1991	3.5
1992	3.4
1993	3.3
1994	3.1
1995	3
1996	3
1997	3
1998	2.9
1999	2.8
2000	2.7
2001	2.6
2002	2.5
2003	2.4
2004	2.4
2005	2.4
2006	2.4
2007	2.4
2008	2.4
2009	2.4
2010	2.3
2011	2.3
2012	2.2
2013	2.1
2014	2
2015	1.9
2016	1.9
2017	1.8
2018	1.8
2019	1.8
2020	1.9
2021	1.9
2022	1.9