Year	per 1,000
1990	7
1991	6.8
1992	6.7
1993	6.6
1994	6.4
1995	6.3
1996	6.2
1997	6
1998	5.9
1999	5.8
2000	5.6
2001	5.5
2002	5.3
2003	5.2
2004	5
2005	4.8
2006	4.7
2007	4.5
2008	4.3
2009	4.2
2010	4
2011	3.8
2012	3.7
2013	3.5
2014	3.3
2015	3.2
2016	3
2017	2.9
2018	2.7
2019	2.6
2020	2.5
2021	2.4
2022	2.2