Year	% of total
1971	31.35016348557097
1972	31.22815576181549
1973	30.23985191248237
1974	30.73189650915078
1975	30.28187007657863
1976	30.325356518875047
1977	28.853595059896474
1978	27.731956566209544
1979	27.123596121394062
1980	27.407985645524764
1981	28.52248134465566
1982	29.325012658659894
1983	29.6532907861414
1984	30.36233056276625
1985	31.827107151309036
1986	32.15355683373253
1987	32.64653703306764
1988	31.28205960830723
1989	32.178840708798056
1990	31.994012563143347
1991	32.33630485821807
1992	32.324820900666076
1993	32.957875505469126
1994	32.73039707364617
1995	33.227023427375904
1996	33.941337105450295
1997	33.7221520742104
1998	32.82264313572521
1999	32.9385736829796
2000	33.18008664183954
2001	33.65015419292614
2002	33.55042208079117
2003	32.95477906192485
2004	32.501680232282226
2005	32.25080335039562
2006	32.31898605187865
2007	32.343273785917596
2008	31.95226873709706
2009	31.998752403807472
2010	32.18658148672672
2011	32.80033758187393
2012	34.37531306324818
2013	34.6783612363646
2014	35.86409102736515
2015	36.43981978529576