Year	per 1,000
1990	5.2
1991	5
1992	4.8
1993	4.7
1994	4.6
1995	4.5
1996	4.5
1997	4.5
1998	4.6
1999	4.6
2000	4.6
2001	4.6
2002	4.6
2003	4.6
2004	4.5
2005	4.5
2006	4.5
2007	4.4
2008	4.4
2009	4.4
2010	4.3
2011	4.3
2012	4.2
2013	4.1
2014	4
2015	3.9
2016	3.8
2017	3.7
2018	3.6
2019	3.5
2020	3.4
2021	3.3
2022	3.2