Year	per 1,000
1990	4.2
1991	4.1
1992	4
1993	3.9
1994	3.8
1995	3.7
1996	3.7
1997	3.6
1998	3.5
1999	3.5
2000	3.5
2001	3.4
2002	3.4
2003	3.4
2004	3.3
2005	3.3
2006	3.3
2007	3.7
2008	3.1
2009	3.1
2010	3
2011	2.9
2012	2.8
2013	2.8
2014	2.7
2015	2.6
2016	2.5
2017	2.5
2018	2.4
2019	2.3
2020	2.8
2021	2.2
2022	2.1