Year	Number
1970	28.01019
1999	25.94511
2000	24.79699
2001	22.97087
2002	22.35331
2003	21.56753
2004	21.38676
2005	21.04365
2007	23.85828
2008	23.02467
2009	22.64411
2010	22.16609
2011	21.28507
2012	20.51712
2013	21.22749
2014	20.91814
2015	20.58529
2016	20.31899
2017	20.22308