Year	per 1,000
1990	4
1991	4
1992	3.9
1993	3.8
1994	3.7
1995	3.6
1996	3.5
1997	3.5
1998	3.5
1999	3.5
2000	3.5
2001	3.5
2002	3.4
2003	3.3
2004	3.3
2005	3.2
2006	3.1
2007	2.9
2008	2.8
2009	2.6
2010	2.5
2011	2.3
2012	2.2
2013	2.2
2014	2.1
2015	2.1
2016	2.1
2017	2.1
2018	2.2
2019	2.2
2020	2.3
2021	2.3
2022	2.4