Year	% of total
1971	100
1972	100
1973	85.78037312300926
1974	81.9115505480187
1975	83.54059985369422
1976	81.01386246262572
1977	85.43103448275862
1978	81.92639299961395
1979	78.96504167416202
1980	68.90998944971959
1981	63.19754614432546
1982	60.54313099041534
1983	58.52521674279855
1984	49.7780925401322
1985	49.60906615127822
1986	44.74587584628588
1987	55.15855489797721
1988	53.19695179102653
1989	52.31689672701677
1990	54.5751509856801
1991	57.491520137237316
1992	60.80402010050251
1993	59.75394108130479
1994	60.02100357717174
1995	63.15616993386165
1996	63.06804258447464
1997	66.56531899389401
1998	66.37644908365
1999	55.50782004247924
2000	57.10973724884081
2001	62.70775770456961
2002	64.35719149111767
2003	66.58103513411409
2004	66.2687420698036
2005	67.63484010111902
2006	63.971611221668454
2007	68.40012749324788
2008	66.08408280034857
2009	67.4149319293939
2010	73.69135838918248
2011	71.3123048456112
2012	71.53083473896409
2013	73.55645311295937
2014	74.35996997230204
2015	74.56347906276922