Year	per 1,000
1990	3.7
1991	3.8
1992	3.9
1993	4
1994	4.2
1995	4.3
1996	4.4
1997	4.5
1998	4.5
1999	4.4
2000	4.3
2001	4.2
2002	4.2
2003	4.1
2004	4.1
2005	4
2006	4.1
2007	4.4
2008	4.6
2009	4.5
2010	4.3
2011	4.1
2012	3.9
2013	3.9
2014	4.1
2015	4.4
2016	4.7
2017	4.9
2018	4.9
2019	4.8
2020	4.7
2021	4.7
2022	4.9