Year	per 1,000
1990	17.4
1991	17
1992	16.6
1993	16.3
1994	16.1
1995	15.9
1996	15.7
1997	15.6
1998	15.5
1999	15.5
2000	15.4
2001	15.5
2002	15.5
2003	15.5
2004	15.6
2005	15.6
2006	15.7
2007	15.7
2008	15.7
2009	15.8
2010	15.8
2011	15.9
2012	15.9
2013	15.8
2014	15.7
2015	15.5
2016	15.3
2017	15.1
2018	14.8
2019	14.6
2020	14.3
2021	14.1
2022	13.8