Year	per 1,000
1990	4.8
1991	5.2
1992	5.5
1993	5.7
1994	5.9
1995	6
1996	6.1
1997	7.6
1998	6
1999	5.7
2000	5.3
2001	4.9
2002	4.5
2003	4.2
2004	3.9
2005	3.8
2006	3.6
2007	3.5
2008	3.4
2009	3.3
2010	3.2
2011	3.1
2012	3
2013	2.8
2014	2.6
2015	2.4
2016	2.3
2017	2.2
2018	2.1
2019	2
2020	2
2021	1.9
2022	1.9