Period	Percent
1970-01-01	7.40
1975-01-01	9.10
1980-01-01	11.90
1985-01-01	14.72
1990-01-01	18.34
1995-01-01	20.49
2000-01-01	24.45
2005-01-01	27.36
2010-01-01	29.36