Year	per 1,000
1990	4.7
1991	4.6
1992	5.1
1993	4.5
1994	4.4
1995	4.3
1996	4.1
1997	4
1998	3.8
1999	3.7
2000	3.5
2001	3.4
2002	3.2
2003	3.1
2004	3
2005	3
2006	2.9
2007	2.9
2008	2.9
2009	2.8
2010	2.8
2011	2.8
2012	2.8
2013	2.8
2014	2.8
2015	2.8
2016	2.9
2017	2.9
2018	3
2019	3
2020	3.1
2021	3.1
2022	3.1