Year	% of total
1970	6.646649793140331
1971	6.798269763861706
1972	6.653038372913637
1973	6.879025815830535
1974	6.876180133010293
1975	7.086322633919581
1976	7.41439286275307
1977	7.174818130678521
1978	6.63903254714396
1979	6.536501343809611
1980	6.660753650886381
1981	7.037923824028244
1982	6.973291104275221
1983	7.141634968523718
1984	7.738244603726736
1985	7.937692465469986
1986	7.883113683463322
1987	7.889338157557553
1988	8.34174790680426
1989	8.37467449830343
1990	8.940926024481106
1991	9.172377438060094
1992	9.653333333333332
1993	9.714285714285714
1994	10.083594566353186
1995	10.503978779840848
1996	10.22380467955239
1997	10.371123538383324
1998	10.449268046441192
1999	10.781808891159939
2000	10.923940786115365
2001	11.544544016868741
2002	11.188811188811188
2003	12.142099681866384
2004	12.54711900915455
2005	12.673056443024494
2006	13.120950323974082
2007	13.052631578947368
2008	13.061435209086216