Year	per 1,000
1990	2.4
1991	2.3
1992	2.3
1993	2.2
1994	2.2
1995	2.1
1996	2.1
1997	2.2
1998	2.2
1999	2.3
2000	2.3
2001	2.3
2002	2.3
2003	2.3
2004	2.2
2005	2.2
2006	2.1
2007	2
2008	1.9
2009	1.7
2010	1.6
2011	1.5
2012	1.5
2013	1.4
2014	1.4
2015	1.4
2016	1.4
2017	1.5
2018	1.5
2019	1.6
2020	1.7
2021	1.8
2022	2