Year	per 1,000
1990	4.1
1991	4.2
1992	4.5
1993	4.6
1994	4.5
1995	4.4
1996	4.2
1997	4.1
1998	4
1999	4.1
2000	3.9
2001	3.8
2002	3.6
2003	3.5
2004	3.3
2005	3.2
2006	3.1
2007	3
2008	2.8
2009	2.6
2010	2.5
2011	2.4
2012	2.3
2013	2.2
2014	2.1
2015	2
2016	2
2017	1.9
2018	1.9
2019	1.8
2020	1.9
2021	1.9
2022	2.1