Year	% of revenue
1972	13.461538461538462
1973	11.538461538461538
1974	11.940298507462686
1975	11.235955056179774
1976	11.403508771929824
1977	10.062893081761008
1978	7.224334600760455
1979	10.163339382940109
1980	7.654563297350343
1981	9.258043335521995
1982	9.496981891348089
1983	10.085607276618513
1984	16.715511827506806
1985	11.447269606265422
1986	9.716536478502356
1987	6.738528242803805
1988	7.444168734491314
1989	4.453927190292039
1990	6.558026284909646
1991	7.384777100964908
1992	8.635586421741142
1993	8.471483539162985
1994	12.646785472151418
1995	11.16577682734577
1996	13.660025130138214
1997	12.876013489025498
1998	14.84787454538344
1999	17.19669694467382
2000	20.444695107372965
2001	15.37742610771334
2002	16.021996086585823
2003	13.353610675039246
2004	11.621731870893855
2005	15.094022399834458
2006	13.802996321130005
2007	14.377382492081455
2008	13.244629324785247
2009	13.880104644396358
2010	14.782645374287414
2011	14.630512950628704
2012	14.042440173562937
2013	14.25556758692361
2014	13.581354694443526
2015	13.683253523093875
2016	13.397790335891186
2017	15.808891378926814
2018	14.551229212419962
2019	16.20664030743241
2020	16.872442195247544
2021	15.236109081956842