Table:
highest certificate, diploma or degree
female lone parents
female parent in couples

1991
2001
2011
difference
1991
2001
2011
difference

percent

total
100
100
100
..
100
100
100
..

no certificate, diploma or degree
34
20
13
-21
24
13
8
-16

high school diploma or equivalency
30
28
25
-5
32
28
21
-11

postsecondary certificate below the bachelor's level
26
39
42
16
29
36
38
9

university degree at the bachelor's level or above
9
13
20
11
15
23
33
18

Statement: the proportion of female lone parents that had a university degree was 9% in 1991 while it had increased 11% to 20% in 2011.
Question: how much did the proportion of female lone parents that had a university degree increase from 1991 to 2011?
Answer: difference