Year	per 1,000
1990	15.7
1991	15.9
1992	16.2
1993	16.5
1994	16.7
1995	16.9
1996	17.2
1997	17.4
1998	17.7
1999	17.9
2000	18.2
2001	18.4
2002	18.6
2003	18.7
2004	18.8
2005	18.9
2006	19
2007	19.1
2008	19.1
2009	19.2
2010	19.2
2011	19.3
2012	19.3
2013	19.3
2014	19.4
2015	19.3
2016	19.2
2017	19.1
2018	18.9
2019	18.8
2020	18.5
2021	18.3
2022	18.1