Year	per 1,000
1990	3.9
1991	4
1992	4.1
1993	4
1994	4
1995	3.8
1996	3.7
1997	3.6
1998	3.5
1999	3.5
2000	3.4
2001	3.4
2002	3.3
2003	3.2
2004	3.2
2005	3.1
2006	3
2007	2.9
2008	2.8
2009	2.8
2010	2.7
2011	2.6
2012	2.6
2013	2.5
2014	2.5
2015	2.4
2016	2.4
2017	2.3
2018	2.3
2019	2.3
2020	2.3
2021	2.3
2022	2.3