Year	per 1,000
1990	4.3
1991	4.1
1992	4
1993	3.9
1994	3.7
1995	3.6
1996	3.5
1997	3.4
1998	3.3
1999	3.3
2000	3.2
2001	3.1
2002	3.1
2003	3.1
2004	3
2005	3
2006	3
2007	3
2008	3
2009	3
2010	3
2011	3
2012	3
2013	3
2014	3
2015	3
2016	2.9
2017	2.9
2018	2.8
2019	2.8
2020	2.7
2021	2.6
2022	2.6