Year	% of total
1971	14.306921761687818
1972	16.49269275674642
1973	18.30146417365676
1974	16.113583277803816
1975	15.640841453242505
1976	13.672636288999879
1977	15.370348529102507
1978	18.952929552182642
1979	21.94009973022897
1980	28.650729103899046
1981	30.78553648145293
1982	31.799382441540956
1983	32.17397896125306
1984	30.60597298784845
1985	29.930069767486383
1986	29.43530393069412
1987	27.002530525882975
1988	24.645688810545437
1989	23.971592282827725
1990	31.62841670008813
1991	28.847423391797598
1992	31.60798811738152
1993	30.97642038688534
1994	31.261457603252495
1995	30.556792337985044
1996	30.689746699393915
1997	30.333282438305247
1998	31.445634563265397
1999	31.997124372188544
2000	32.27985022325271
2001	29.647568914376137
2002	30.725410564680004
2003	31.29740799196801
2004	30.308929433479264
2005	24.10745596146231
2006	22.32181060141735
2007	22.844584267445303
2008	23.641440421982235
2009	22.05236921771184
2010	22.704437976910416
2011	21.34048068067998
2012	21.627481329788253
2013	20.861812480018216
2014	20.00606818412784
2015	19.593454308352424