Table:
characteristic
prevalence
mean vitamin c concentration

%
95% confidence interval
μmol/l
95% confidence interval

from
to
from
to

total
100.0
..
..
53
50
56

sex

men
49.8
48.9
50.7
47
43
52

women
50.2
49.3
51.1
59
55
63

age group

20 to 39
37.1
36.0
38.1
54
50
58

40 to 59
40.1
39.3
40.8
51
47
55

60 to 79
22.8
22.3
23.4
56
52
59

race

white
75.9
63.3
85.3
54
51
56

other
24.1
14.7
36.7
52
46
58

household income tertile

lower
32.1
26.6
38.1
50
45
55

middle
30.2
25.1
35.9
54
51
57

higher
37.7
33.1
42.5
55
51
59

education

less than college/university graduation
31.0
26.2
36.3
46
42
49

college/university graduation
69.0
63.7
73.8
56
54
59

smoking

daily/occasional
20.0
16.0
24.7
45
40
50

non-smoker
80.0
75.3
84.0
55
52
58

body mass index

neither overweight nor obese
38.1
32.1
44.5
58
54
63

overweight
36.3
31.2
41.8
53
49
58

obese
25.6
20.0
32.1
45
42
49

vitamin c-containing supplement use

no
78.2
71.1
83.9
49
46
52

yes
21.8
16.1
28.9
69
65
73

fruit juice consumption

daily
24.5
19.7
30.0
59
55
62

at least once a week
39.4
33.9
45.1
53
49
57

at least once a month
15.8
11.8
20.9
51
46
57

less than once a month or never
20.4
17.7
23.3
49
43
55

fruit drink consumption

at least once a week
9.2
7.4
11.4
49
43
56

at least once a month
8.2
4.1
15.6
45
36
55

less than once a month or never
82.6
75.5
88.0
54
52
57

citrus fruit consumption

daily
16.0
12.3
20.6
58
52
64

at least once a week
44.3
38.6
50.1
55
52
58

at least once a month
21.9
18.4
25.8
51
46
56

less than once a month or never
17.9
13.9
22.6
46
39
52

Statement: users of vitamin c-containing supplements had mean concentrations that were 20 μmol/l higher than those of non-users.
Question: what was the difference between users of vitamin c-containing supplements of mean concentrations and those of non-users?
Answer: [20]