Period	Percent
1970-01-01	2.60
1975-01-01	2.95
1980-01-01	3.30
1985-01-01	4.28
1990-01-01	6.03
1995-01-01	8.23
2000-01-01	11.61
2005-01-01	15.42
2010-01-01	17.39