Period	Percent
1970-01-01	3.73
1975-01-01	4.53
1980-01-01	5.21
1985-01-01	5.87
1990-01-01	6.36
1995-01-01	7.60
2000-01-01	9.84
2005-01-01	13.46
2010-01-01	18.29