Year	per 1,000
1990	3.7
1991	3.6
1992	3.5
1993	3.4
1994	3.3
1995	3.2
1996	3.1
1997	3.1
1998	3
1999	2.9
2000	2.9
2001	2.8
2002	2.7
2003	2.7
2004	2.6
2005	2.6
2006	2.6
2007	2.5
2008	2.5
2009	2.4
2010	2.4
2011	2.4
2012	2.3
2013	2.3
2014	2.3
2015	2.2
2016	2.2
2017	2.1
2018	2.1
2019	2
2020	2
2021	2
2022	1.9