Year	per 1,000
1990	4.8
1991	4.6
1992	4.5
1993	4.3
1994	4.2
1995	4
1996	3.9
1997	3.8
1998	3.7
1999	3.6
2000	3.5
2001	3.4
2002	3.3
2003	3.3
2004	3.2
2005	3.2
2006	3.2
2007	3.2
2008	3.2
2009	3.2
2010	3.2
2011	3.1
2012	3.1
2013	3
2014	3
2015	2.9
2016	2.8
2017	2.8
2018	2.7
2019	2.7
2020	2.6
2021	2.5
2022	2.5