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