5.2
Comparison of participants with nonparticipants
Participation bias can occur if participants differ from non-participants on characteristics
which are associated with the study dependent measures, such as health status. A
complete examination of participation bias would require the collection of comprehensive
and current health, demographics and deployment information for all non-participating Gulf
War veterans and comparison group subjects. Whilst such data are not available for nonparticipants, we are able to conduct a number of comparisons of participants and nonparticipants using data which were collected at the time of the baseline study, to assess the
extent to which participants are representative of the study groups from which they were
drawn.
Table 15 shows the comparison of participants with non-participants in each study group,
across a number of demographic, service and health-related variables using data from the
baseline study. Participating Gulf War veterans were older and more likely to have been
Officers in 1991 compared to veterans who did not participate. Participating Gulf War
veterans had very slightly lower median SF-12 mental health scores (representing poorer
mental health) than non-participating veterans, with this difference only just achieving
statistical significance. There were no statistically significant differences observed between
participating, and non-participating, Gulf War veterans in regard to their gender, service
branch, whether or not they were still serving in the ADF at baseline, their SF-12 physical
health score at the time of the baseline study, and whether or not they met CIDI criteria or
not for PTSD, major depression or alcohol dependence in the 12 months prior to the
baseline study. Participating comparison group members were older, more likely to have
been in the Air Force and more likely to have been Officers in 1991, relative to comparison
group members who did not participate. There were no statistically significant differences
observed between participating, and non-participating, comparison group members across
the remaining demographic and health variables included in the table.
The differences observed between participants and non-participants in the two groups may
mean that the study slightly over-estimates or underestimates the true health of each group;
i.e. the observed health results for each group may vary slightly from that which would have
been observed if full participation has occurred. However, because a similar pattern of nonparticipation amongst the younger and lower ranked group members occurred in both study
groups, it is unlikely that those two factors will notably affect the magnitude or direction of
any differences in health observed between the Gulf War veteran and the comparison group.
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The observed difference between participating and non-participating Gulf War veterans in
median SF-12 mental health is so small that it is also unlikely to have an impact on the
magnitude of differences in health observed between the participating groups. The
increased participation rate by Air Force members in the comparison group may contribute
to the study slightly overestimating the true health of the comparison group; and this
highlights the need to statistically adjust for service type when comparing the health
outcomes across the two groups.
Table 15 Comparison of participants with non-participants in the follow up study
Gulf War veterans
Measure
n (%)
Nonparticipants
(N=719)†
n (%)
697 (50.0)
702 (50.2)
Participants
(N=712)*
Comparison group
n (%)
Nonparticipants
(N=899)†
n (%)
659 (43.0)
874 (57.0)
Participants
(N=674)*
χ2 p-value
χ2 p-value
Gender
male
0.742
female
0.489
15 (46.9)
17 (53.1)
15 (37.5)
25 (62.5)
< 40
34 (37.0)
58 (63.0)
19 (32.8)
39 (67.2)
40 - 44
171 (39.8)
259 (60.2)
118 (28.9)
290 (71.1)
45 - 49
208 (53.6)
180 (46.4)
185 (41.9)
257 (58.1)
50 - 54
172 (55.3)
139 (44.7)
189 (51.5)
178 (48.5)
>= 55
127 (60.5)
83 (39.5)
163 (54.7)
135 (45.3)
457 (40.5)
671 (59.5)
75 (42.4)
102 (57.6)
Age at follow up‡
<0.001
<0.001
Service branch in 1991
Navy
606 (49.3)
624 (50.7)
Army
51 (55.4)
41 (44.6)
Air Force
55 (50.5)
54 (49.5)
142 (53.0)
126 (47.0)
157 (57.7)
115 (42.3)
205 (51.1)
196 (48.9)
339 (45.8)
401 (54.2)
130 (30.1)
302 (69.9)
277 (44.0)
352 (56.0)
397 (42.1)
547 (57.9)
606 (44.1)
768 (55.9)
10 (45.5)
12 (54.6)
585 (44.3)
735 (55.7)
31 (40.8)
45 (59.2)
0.515
0.001
Rank in 1991
Officer
Other-ranks
358 (52.3)
327 (47.7)
supervisory
Other ranksnon
196 (41.5)
276 (58.5)
supervisory
Still serving in the ADF at baseline
Yes
291 (48.0)
<0.001
315 (52.0)
0.260
No
421 (51.0)
404 (49.0)
<0.001
0.436
CIDI diagnosis in 12 months preceding baseline§
PTSD
absent
667 (50.7)
650 (49.4)
0.300
present
31 (44.3)
39 (55.7)
644 (50.9)
621 (49.1)
0.899
Major depression
absent
0.161
present
54 (44.3)
68 (55.7)
Australian Gulf War Veterans’ Follow Up Health Study: Technical Report 2015
0.547
Page 77
Gulf War veterans
Measure
Participants
(N=712)*
n (%)
Nonparticipants
(N=719)†
n (%)
Comparison group
n (%)
Nonparticipants
(N=899)†
n (%)
605 (44.4)
758 (55.6)
11 (33.3)
22 (66.7)
Median
(IQR)
Participants
(N=674)*
χ2 p-value
χ2 p-value
Alcohol dependence/abuse
absent
674 (50.8)
653 (49.2)
0.102
present
SF-12 at baseline
Physical
Health
Component
Mental Health
Component
0.206
24 (40.0)
36 (60.0)
Median
(IQR)
Median
(IQR)
Wilcoxon
test p-value
Median
(IQR)
52.4
(45.3-55.9)
52.2
(44.4 - 55.6)
0.611
53.4
(46.9-55.9)
53.1
(46.0-55.9)
0.345
51.4
(41.1 – 56.5)
50.6
(38.5 - 55.9)
0.059
55.0
(47.6-57.8)
53.7
(45.8-57.4)
0.012
Wilcoxon
test p-value
* Includes all those who completed the health questionnaire at follow up
† Excludes those who are known to be deceased
‡ Age was calculated as of 01 June 2011
§ Only calculated for those with baseline CIDI results
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