Year	per 1,000
1990	6.9
1991	6.8
1992	6.6
1993	6.5
1994	6.4
1995	6.4
1996	6.4
1997	6.4
1998	6.4
1999	6.4
2000	6.5
2001	6.5
2002	6.5
2003	6.4
2004	6.4
2005	6.4
2006	6.3
2007	6.3
2008	6.3
2009	6.2
2010	6.2
2011	6.2
2012	6.1
2013	6
2014	6
2015	5.9
2016	5.8
2017	5.7
2018	5.6
2019	5.5
2020	5.4
2021	5.3
2022	5.2