Year	per 1,000
1990	3.9
1991	3.9
1992	4
1993	4
1994	4
1995	4.1
1996	4.1
1997	4.1
1998	4.1
1999	4.2
2000	4.2
2001	4.2
2002	4.2
2003	4.2
2004	4.2
2005	4.3
2006	4.4
2007	4.5
2008	4.6
2009	4.7
2010	4.9
2011	5.1
2012	5.2
2013	5.3
2014	5.4
2015	5.5
2016	5.6
2017	5.7
2018	5.8
2019	5.9
2020	6
2021	6
2022	6