Period	Percent
1970-01-01	11.89
1975-01-01	20.10
1980-01-01	26.50
1985-01-01	33.29
1990-01-01	39.07
1995-01-01	37.90
2000-01-01	38.64
2005-01-01	40.07
2010-01-01	36.81