Year	per 1,000
1990	4.3
1991	4.2
1992	4.2
1993	4.2
1994	4.1
1995	4
1996	3.8
1997	3.6
1998	3.5
1999	3.4
2000	3.3
2001	3.3
2002	3.2
2003	3.2
2004	3.2
2005	3.2
2006	3.1
2007	3
2008	2.8
2009	2.6
2010	2.5
2011	2.3
2012	2.3
2013	2.3
2014	2.3
2015	2.3
2016	2.4
2017	2.4
2018	2.5
2019	2.5
2020	2.7
2021	3
2022	3.2