Year	per 1,000
1990	22.3
1991	22.2
1992	22.1
1993	22
1994	21.9
1995	21.7
1996	21.6
1997	21.4
1998	21.2
1999	20.9
2000	20.6
2001	20.3
2002	19.9
2003	19.6
2004	19.2
2005	18.9
2006	18.5
2007	18.1
2008	17.7
2009	17.3
2010	17
2011	16.6
2012	16.3
2013	16
2014	15.6
2015	15.3
2016	15
2017	14.8
2018	14.4
2019	14.2
2020	13.9
2021	13.6
2022	13.3