Year	Number
1971	22.6097
1972	20.62546
1973	23.46615
1975	20.43593
1976	19.83741
1983	22.47071
1999	27.86887
2003	31.34198
2006	29.60091
2007	28.19459
2008	30.33107
2009	25.62369
2010	30.25804
2011	26.48814
2012	26.33301
2013	26.90824
2014	27.08945
2015	25.19571
2016	23.38275
2017	23.33086
2018	23.11995