Year	per 1,000
1990	3.6
1991	3.7
1992	3.7
1993	3.8
1994	3.9
1995	4
1996	4.2
1997	4.3
1998	4.3
1999	4.3
2000	4.1
2001	4
2002	3.8
2003	3.6
2004	3.5
2005	3.4
2006	3.3
2007	3.2
2008	3.1
2009	3
2010	2.9
2011	2.7
2012	2.6
2013	2.4
2014	2.3
2015	2.1
2016	1.9
2017	1.8
2018	1.6
2019	1.5
2020	1.4
2021	1.4
2022	1.3