Year	per 1,000
1990	3.6
1991	3.7
1992	3.9
1993	4
1994	4.2
1995	4.3
1996	4.4
1997	4.4
1998	4.4
1999	4.4
2000	4.4
2001	4.3
2002	4.2
2003	4
2004	3.8
2005	3.5
2006	3.3
2007	3
2008	2.8
2009	2.6
2010	2.4
2011	2.3
2012	2.2
2013	2.1
2014	2.1
2015	2.1
2016	2
2017	2
2018	2
2019	2
2020	1.9
2021	1.9
2022	1.8