MilCohort Assessing nonresponse bias at follow-up_BMC Med Res Methodol 2010

Littman et al. BMC Medical Research Methodology 2010, 10:99
http://www.biomedcentral.com/1471-2288/10/99

RESEARCH ARTICLE

Open Access

Assessing nonresponse bias at follow-up in a
large prospective cohort of relatively young and
mobile military service members
Alyson J Littman1,2*, Edward J Boyko1, Isabel G Jacobson3, Jaime Horton3, Gary D Gackstetter4, Besa Smith3,
Tomoko Hooper5, Timothy S Wells3, Paul J Amoroso6, Tyler C Smith3, the Millennium Cohort Study

Abstract
Background: Nonresponse bias in a longitudinal study could affect the magnitude and direction of measures of
association. We identified sociodemographic, behavioral, military, and health-related predictors of response to the
first follow-up questionnaire in a large military cohort and assessed the extent to which nonresponse biased
measures of association.
Methods: Data are from the baseline and first follow-up survey of the Millennium Cohort Study. Seventy-six
thousand, seven hundred and seventy-five eligible individuals completed the baseline survey and were presumed
alive at the time of follow-up; of these, 54,960 (71.6%) completed the first follow-up survey. Logistic regression
models were used to calculate inverse probability weights using propensity scores.
Results: Characteristics associated with a greater probability of response included female gender, older age, higher
education level, officer rank, active-duty status, and a self-reported history of military exposures. Ever smokers, those
with a history of chronic alcohol consumption or a major depressive disorder, and those separated from the
military at follow-up had a lower probability of response. Nonresponse to the follow-up questionnaire did not
result in appreciable bias; bias was greatest in subgroups with small numbers.
Conclusions: These findings suggest that prospective analyses from this cohort are not substantially biased by
non-response at the first follow-up assessment.

Background
Intragroup comparisons over time are a key strength of
longitudinal cohort studies; a major threat to the validity
of results from such studies is nonresponse to follow-up
surveys and/or attrition, which can result in a loss of
statistical power and bias. When only a subset of all participants provides follow-up information on exposures
and outcomes, the participating subset may not be
representative of the original sample. Prior studies have
found that follow-up responders tend to differ from
nonresponders in their sociodemographic and health
characteristics. Since it may be difficult or impossible to
determine whether nonresponse is related to the
* Correspondence: alyson@u.washington.edu
1
Seattle Epidemiologic Research and Information Center, Department of
Veterans Affairs Puget Sound Health Care System, Seattle, WA, USA
Full list of author information is available at the end of the article

outcome under study, great efforts are usually devoted
to maximizing participation and minimizing dropout.
Despite such efforts, follow-up survey nonresponse is
inevitable and the extent to which such nonresponse
might bias study results is a methodological issue of
high interest and ongoing concern.
Although not entirely consistent, a number of studies
have found that individuals with the following characteristics are more likely to drop out of studies: men (vs.
women), not married (vs. married), current smokers,
lower socioeconomic status, and poorer health [1-4].
Longitudinal studies of elderly adults have noted that
follow-up nonresponders are more likely to be older ([5]
and references therein), while studies of younger adults
have observed the opposite [2,3]. Nevertheless, few studies have investigated factors predicting nonresponse to
follow-up surveys in longitudinal cohorts of younger

© 2010 Littman et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.

Littman et al. BMC Medical Research Methodology 2010, 10:99
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in June 2004, cohort members were recontacted via
e-mail and postal service to complete a follow-up survey. Twenty-nine of the responders to the baseline survey were later determined to be ineligible and 157
individuals died before June 2004. Methods for determining vital status are described in detail elsewhere
[12]. Of the 76,861 individuals presumed alive at the
time of the administration of the follow-up survey
(June 2004-February 2006), 55,046 individuals completed it. After excluding 86 individuals with missing
responses for covariates (see Statistical analyses section), 76,775 individuals were included in analyses to
calculate the propensity score and 54,960 individuals
were included in analyses as responders to the followup survey. All enrolled subjects provided informed
consent. This study was approved by the Institutional
Review Board at the Naval Health Research Center,
San Diego (protocol number NHRC.2000.0007).

adult participants. Furthermore, although bias is a major
concern, most studies that have used various methods
to try to account for nonresponse (e.g., inverse probability weighting and multiple imputation) have not
detected substantial bias in estimated measures of association [6-10].
The Millennium Cohort is a 22-year prospective
cohort study that began enrollment in 2001 and administered its first follow-up assessment in 2004. The
cohort comprises a population of relatively young, highly
mobile men and women, often exposed to unique and
stressful job circumstances. Moreover, extensive information was collected at baseline on mental, physical,
and behavioral health, in addition to sociodemographic,
service-related, and occupational characteristics. Previously published Millennium Cohort studies [11-20]
have included 1) an investigation of differences in early
vs. late responders, 2) a comparison of the cohort to the
overall military population, 3) analyses to adjust health
outcomes based on the inverse of the sampling and
response patterns, 4) evaluation of the early mortality
experience among Millennium Cohort participants and
invited non-participants, and 5) investigations of health
characteristics prior to enrollment. These thorough evaluations of possible biases have demonstrated that
Cohort members are generally representative of the US
military, that health prior to enrollment did not influence participation, and that Cohort questionnaire data
are reliable and internally consistent [11-20]. To complement these previous efforts, the objectives of the current study were to: 1) identify sociodemographic,
behavioral, military, and health-related factors associated
with response to the follow-up questionnaire, and 2)
assess the extent to which failure to account for nonresponse may bias measures of associations between predictors and outcomes under investigation.

A modified Dillman method was used to maximize participation at baseline and follow-up, and it included an
introductory postcard, survey, and reminder postcard
mailings, with repeated survey and reminder postcard
mailings for nonresponders [21]. Semiannual e-mails
and postcards (sent on Memorial Day and Veterans
Day) were used to track participants, sustain interest in
continued participation, and verify accuracy of contact
information [22]. Participants were sent specially
designed messages thanking them for their contribution
to military service and to the study and directing them
to the study Web site to obtain information on study
progress and findings and to update their contact information. In addition, the US Postal Service’s “Return Service Requested” was used to obtain forwarding
addresses on undeliverable postcards.

Methods

Data collection

The Millennium Cohort

Demographic and military data were obtained from the
electronic personnel files of the Defense Manpower
Data Center and included gender; birth date; race/ethnicity; education; marital status; branch of service; service
component; military pay grade; military occupation;
deployment experience to Southwest Asia, Bosnia, or
Kosovo between 1998 and 2000; deployment experience
in support of the wars in Iraq and Afghanistan between
2001 and 2006; and military status at follow-up.
Self-reported data on diagnosed medical conditions,
symptoms, psychosocial assessment, occupation(s), use
of alcohol and tobacco, as well as military-specific and
occupational exposures were obtained from the Millennium Cohort baseline questionnaire, which consisted of
more than 450 questions. More information about the
survey instrument is available elsewhere [18].

The sampling frame and participant recruitment procedures for the Millennium Cohort have been
described in detail elsewhere [18]. Briefly, 256,400 military personnel, representing 11.3 percent of the 2.2
million men and women in active service as of October
1, 2000, were invited to participate in the Millennium
Cohort Study between July 2001 and June 2003.
Female service members, Reserve and National Guard
personnel, and those previously deployed were oversampled. Enrollment was conducted by mail and later
by electronic mail invitations. The e-mail invitations
presented the option to complete the survey using a
Web-based, online questionnaire. A total of 77,047 eligible individuals completed the baseline questionnaire;
over half of the respondents did so online. Beginning

Strategies for maximizing response

Littman et al. BMC Medical Research Methodology 2010, 10:99
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Statistical analyses

To assess differences in terms of demographic, deployment, occupational, and behavioral characteristics
between responders and nonresponders to the follow-up
questionnaire, we first conducted descriptive analyses
using chi-square tests of association. Next, to calculate
propensity scores, we conducted multivariable logistic
regression with response to the follow-up questionnaire
as the outcome variable. In this case, the propensity
score can be thought of as the conditional probability
that a person responds given the set of covariates. We
assumed that data were missing at random (MAR),
meaning that the probability of nonresponse at followup depended only on observed data. We used the likelihood ratio test to compare models including a given
variable versus absence of the variable in the model (the
nested model). Variables with P values > 0.05 were
removed from the model. We considered for inclusion
all variables included in Table 1 as well as the following
variables: deployment to Southwest Asia, Bosnia, or
Kosovo between 1998 and 2000; binge drinking (drinking ≥5 drinks on a single occasion); survey mode (paper
vs. Web); body mass index category (<18.5, 18.5-24.9,
25.0-29.9, ≥30 kg/m2); and panic syndrome, other anxiety, eating disorder, hypertension, diabetes, chronic fatigue syndrome, and posttraumatic stress disorder
(PTSD), which were all coded as yes/no responses. The
final model was comprised of the variables presented in
Table 1. To assess the robustness of our model inclusion criterion, we also compared nested models using
Akaike’s Information Criterion (AIC). The AIC is a calculated index that takes into account both the statistical
goodness of fit and the number of parameters that have
to be estimated to achieve this particular degree of fit by
imposing a penalty for increasing the number of parameters. Lower values of the index indicate the preferred
model, that is, the one with the fewest number of parameters that still provides an adequate fit to the data.
The same factors were retained in our model whether
we determined inclusion based on a P value < 0.05 or a
lower AIC.
To improve the predictive value of our model, we
considered the following first-order interaction terms
for inclusion based on a review of the scientific literature and the strength of associations in the multivariable
analyses (previous step): gender, age, education, and
race/ethnicity, each with the others and with each of the
following: marital status, military pay grade, military status at follow-up, service branch, and self-reported health
status. We also considered interactions between age and
deployment experience and age and self-reported military exposures. We used a two-step process to determine which interaction terms to include in the final
model. First, we added interaction terms to the main

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effects model one at a time. The terms with a P value >
0.05 were dropped from further consideration. In the
second step, we sequentially added the interaction terms
with the smallest P value (or smallest AIC in cases of
the same P value) from the first step. Interaction terms
with P > 0.05 after inclusion of main effects and the
other interaction terms were subsequently removed
from the model.
We used the inverse of the propensity score calculated
from the multivariable logistic regression model
described above to calculate a probability weight for
each person, although only responders to the follow-up
questionnaire were included in subsequent analyses [23].
Individuals with lower propensities for response were
weighted more heavily than those with higher propensities, such that a responder with a propensity equal to
0.2 carried a weight of 5 and a responder with a propensity equal to 0.85 carried a weight of 1.18. The sum of
the assigned weights is equal to the baseline population
(n = 76,775).
To evaluate the extent to which nonresponse may
have influenced measures of association, we used logistic
regression along with survey commands that allowed for
weighting responders according to their propensity
score-derived weights ("weighting for nonresponse”). We
selected three outcomes that had been previously studied using Millennium Cohort Study data: disordered
eating, depression, and PTSD [24-26]. These outcomes
were chosen to include a range of important physical
and mental health outcomes. We used the same exclusion criteria and adjusted for the same factors as in the
original published studies in order to compare nonresponse-weighted results with the published findings
[24-26]. Additionally, to evaluate whether weighting for
nonresponse affected estimates of self-reported health at
follow-up, we conducted a fourth analysis with selfreported health (five-level variable: excellent, very good,
good, fair, poor) as the outcome. For this analysis, we
adjusted the proportions based on the propensity scorederived weights.

Results
Table 1 shows the distribution of demographic and military characteristics ascertained from the DMDC; selfreported military, behavioral, and health characteristics;
and survey response characteristics among responders
and nonresponders. Overall, 71.6 percent (n = 54,960)
of individuals completed the first follow-up survey. The
response proportion was 10 or more percentage points
above average (i.e., ≥81.6 percent) in the following subgroups: age ≥44 years, educational level of a bachelor’s
degree or higher; and rank of warrant or commissioned
officer. The response proportion was 10 or more percentage points below average (≤61.6 percent) among

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Table 1 Characteristics of Millennium Cohort Study participants according to response to follow-up and multivariable
regression coefficients predicting response (N = 76,775)
Characteristic

Follow-up response status
Follow-up
Follow-up
responder nonresponder
N = 54,960 N = 21,815
n

%

n

Multivariable logistic
regression model
Beta (standard
error)

P
value

-0.24 (0.07)

0.0006

<0.0001

%

Intercept
Demographic and military characteristics obtained from the Defense Manpower
Data Center
Gender
Male
Female
Age group (years)

40,311 71.7

15,895

28.3

Ref

14,649 71.2

5,920

28.8

0.36 (0.07)

17-24

7,932 54.7

6,576

45.3

Ref

25-34

19,249 71.3

7,766

28.7

0.56 (0.08)

35-44

19,600 77.4

5,727

22.6

1.03 (0.09)

>44

8,179 82.4

1,746

17.6

1.26 (0.12)

White, non-Hispanic

38,965 72.9

14,468

27.1

Ref

Black, non-Hispanic
Asian/Pacific Islander

6,721 63.5
4,859 80.1

3,859
1,207

36.5
19.9

-0.49 (0.03)
-0.02 (0.05)

<0.0001

Race/ethnicity

Native American
Hispanic
Other

453

66.8

3,202 64.9
760

69.9

225

33.2

-0.10 (0.14)

1,728

35.1

-0.30 (0.05)

328

30.1

-0.19 (0.09)

<0.0001

Education
Less than high school

2,957 62.9

1,743

37.1

Ref

High school diploma or equivalent

21,053 64.1

11,802

35.9

0.28 (0.07)

Some college
Bachelor’s degree

14,593 74.4
10,353 81.7

5,011
2,312

25.6
18.3

0.66 (0.08)
0.76 (0.16)

Postgraduate

6,004 86.4

947

13.6

2.37 (1.07)

<0.0001

Marital status
Never married

14,548 63.0

8,541

37.0

Ref

Married

36,484 75.3

11,949

24.7

0.10 (0.02)

Divorced/widowed/separated

3,928 74.8

1,325

25.2

0.04 (0.04)

26,261 72.2
9,918 70.0

10,118
4,259

27.8
30.0

Ref
-0.20 (0.03)

<0.0001

Branch of service
Army
Navy/Coast Guard
Marines

2,257 57.6

1,659

42.4

-0.43 (0.05)

Air Force

16,524 74.1

5,782

25.9

-0.24 (0.03)

<0.0001

Service component
Reserve/Guard

24,084 73.0

8,928

27.0

Ref

Active duty

30,876 70.6

12,887

29.4

0.44 (0.02)

Enlisted
Warrant officer

40,089 67.8
1,168 84.5

19,046
214

32.2
15.5

Ref
0.23 (0.10)

Commissioned officer

13,703 84.3

2,555

15.7

0.20 (0.07)

<0.0001

Military pay grade
0.0005

Occupational category
Combat specialists

11,212 73.0

4,153

27.0

Ref

Electronic equipment repair

5,050 74.6

1,717

25.4

0.21 (0.04)

Communications/intelligence

3,894 71.9

1,523

28.1

0.16 (0.04)

Health care

6,144 77.0

1,837

23.0

0.14 (0.04)

Other technical and allied specialists

1,379 70.0

590

30.0

0.08 (0.06)

<0.0001

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Table 1 Characteristics of Millennium Cohort Study participants according to response to follow-up and multivariable
regression coefficients predicting response (N = 76,775) (Continued)
Functional support and administration

11,122 72.4

4,236

27.6

0.11 (0.03)

Electrical/mechanical equipment repair

7,627 67.2

3,725

32.8

0.05 (0.03)

Craft workers

1,635 68.6

748

31.4

0.08 (0.05)

Service and supply

4,680 70.1

1,993

29.9

0.06 (0.04)

Students, trainees, and other

2,217 63.2

1,293

36.8

0.02 (0.04)

40,823 70.8
14,137 73.8

16,799
5,016

29.2
26.2

Ref
0.08 (0.02)

<0.0001

Enlisted

34,324 71.7

13,560

28.3

Ref

<0.0001

Officer

14,727 84.5

2,706

15.5

0.76 (0.18)

No longer in military, retired

3,848 63.6

2,203

36.4

-1.51 (1.24)

No longer in military, other

2,120 38.6

3,373

61.4

-1.03 (0.15)

Deployment experience between 2001 and 2006a
None
Deployed
Military status at follow-up

Self-reported military, behavioral, and health characteristics
Self-reported military exposures
No reported exposures

39,197 70.9

16,074

29.1

Ref

Witnessed a person’s death due to war, disaster, or tragic event

12,708 73.2

4,652

26.8

0.05 (0.02)

Chemical or biological warfare agents

1,427 72.6

538

27.4

0.12 (0.06)

Both

1,628 74.7

551

25.3

0.09 (0.05)

No

51,186 72.1

19,811

27.9

Ref

Yes

3,774 65.3

2,004

34.7

-0.07 (0.03)

0.01

Chronic drinking

Smoking status
Nonsmoker

32,102 74.0

11,287

26.0

Ref

Ever/past smoker

13,594 72.8

5,071

27.2

-0.06 (0.02)

Current smoker

8,332 64.7

4,538

35.3

-0.22 (0.02)

919

49.6

-0.97 (0.05)

Unknown

932

50.4

0.0192

<0.0001

Self-reported general health
Excellent

11,248 75.0

3,746

25.0

Ref

Very good

22,233 73.3

8,102

26.7

0.06 (0.02)

Good
Fair

16,222 69.6
3,467 65.4

7,077
1,837

30.4
34.6

0.04 (0.03)
0.02 (0.04)

Poor

57.9

233

42.1

-0.14 (0.10)

1,470 64.2

820

35.8

-0.24 (0.05)

No

53,431 72.0

20,818

28.0

Ref

Yes

1,529 60.5

997

39.5

-0.16 (0.05)

37,656 68.9

17,019

31.1

Ref

17,304 78.3

4,796

21.7

0.46 (0.02)

Unknown

320

<0.0001

Major depressive disorder
0.0010

Survey response characteristics
Early response to baseline questionnaireb
No
Yes

<0.0001

Interaction terms
Age group × military status at follow-up interactionc

<0.0001

Education × military status at follow-up interactionc

<0.0001

Gender × educationc

0.0012

Age group × educationc

0.0055

Gender × branch of servicec
Race/ethnicity × branch of servicec

0.0239
0.0021

a

Deployment experience from 2001 to 2006 refers to deployments in support of the wars in Iraq and Afghanistan.
Early response was defined as completing the questionnaire prior to September 1, 2001, which was within 2 months of enrollment invitation on July 1, 2001.
c
Each interaction generated multiple terms. The number of these terms ranged from 4 (gender × education) to 15 (race/ethnicity × branch of service).
b

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individuals who were aged 17-24 years, no longer in the
military at follow-up for reasons other than retirement,
Marine Corps service members, missing smoking status,
and those who reported having poor health or a major
depressive disorder.
Table 1 also includes the adjusted beta coefficients
and P values for the full propensity score model for
each stratum compared with its reference category. A
coefficient <0 indicates that the multivariable-adjusted
probability of response is smaller than the reference
category (equivalent to an odds ratio [OR] <1). Conversely, coefficients >0 indicate that the multivariable
adjusted probability of response is larger than the reference category (equivalent to OR >1). To describe an
individual’s estimated multivariable response probability,
beta coefficients across the various characteristics are
summed. Since the focus of this analysis was on development of the propensity scores, and not specific ORs,
coefficients for each interaction term are not shown in
Table 1; the following example is provided for illustration. Women in the Army with less than a high school
education (the reference category) were more likely to

Figure 1 Propensity score statistics (N = 54,960 responders)

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respond than men in the Army with less than a high
school education (OR = exp[0.36] = 1.43). In contrast,
women in the Army with a bachelor’s degree were no
more likely to respond than men in the Army with the
same level of education (OR = exp[0.36 +(-0.33)] =
1.03).
Figure 1 presents information about the distribution of
propensity scores. The mean propensity score was 0.75,
indicating that the average weight given to each followup responder was 1.33, while the minimum and maximum weights were 1.03 and 14.4, (corresponding to
maximum and minimum propensity scores of 0.97 and
0.0696, respectively). The C statistic, a measure of the
goodness of fit for the model, was equal to 0.71.
Tables 2, 3, and 4 present ORs and 95 percent confidence intervals (CIs) for the complete case analysis (i.e.,
“unweighted” results, ignoring nonresponse) and
weighted for nonresponse for the association between
new-onset eating disorders (Table 2), depression (Table
3), and PTSD (Table 4), respectively, and various exposures, including deployment experience, history of alcohol misuse, and smoking status. In the unweighted

Littman et al. BMC Medical Research Methodology 2010, 10:99
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analyses, the following characteristics were associated
with increased risks of new-onset eating disorders
(Table 2): a history of diagnosed mental disorders (men
and women), being on a special diet for weight loss
(men and women), being active duty (vs. Reserve/Guard,
men only), and a history of major life stressors or alcohol misuse (both for men only). After weighting for
nonresponse, there was little change in ORs for any
strata and no change in the interpretation of results. In
some cases, the 95 percent CIs were slightly wider for
the nonresponse-weighted estimates. Associations
between new-onset depression and deployment experience, smoking status, problem drinking, and PTSD at
baseline among men and women were similar with and
without weighting for nonresponse (Table 3), although
95 percent CIs were again slightly wider in some cases
after weighting for nonresponse.
As in the published study by Smith et al. [26], ORs for
the associations of deployment experience, gender,
smoking status, problem drinking, military rank, and
new-onset PTSD were stratified by service branch
(Army, Air Force, Navy and Coast Guard, and Marines;
Table 4). The number of new-onset PTSD cases by

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service branch was 906 for Army, 184 for Air Force, 195
for Navy and Coast Guard, and 62 for Marine Corps.
Particularly for Marines, the sample sizes were small
and resulted in relatively imprecise OR estimates. In
Army, Navy, and Coast Guard members, deployment
without combat exposures was associated with statistically significant reductions in new-onset PTSD in the
nonresponse weighted analyses, but not in the complete
case analyses (nonresponse weighted analyses: Army,
OR: 0.63, 95 percent CI: 0.44, 0.92; Navy/Coast Guard,
OR: 0.48, 95 percent CI: 0.25, 0.90). Weighting for nonresponse in analyses of Marines resulted in a shift in
ORs from greater than one to less than one, but the 95
percent CIs for both the unweighted and weighted analyses included the null value. For all service branches,
the associations between deployment with combat exposure and risk of new-onset PTSD remained strong and
positive after weighting. In the unweighted analyses,
problem drinking in Marines was associated with a 73
percent increased risk of PTSD, which was of borderline
statistical significance (OR = 1.73, 95 percent CI: 1.00,
2.99); after weighting for nonresponse, the point estimate was attenuated toward the null and the confidence

Table 2 Comparison of associations of new-onset eating disorders based on complete case results (ignoring
nonresponse) and weighted for nonresponse
Adjusted odds ratio (95% CI) of new-onset eating disorders
Women (N = 12,641)

Men (N = 33,577)

Complete case
analysis

Weighted for
nonresponse

Complete case
analysis

Weighted for
nonresponse

1.00
0.83 (0.56, 1.23)

1.00
0.89 (0.60, 1.32)

1.00
0.91 (0.73, 1.13)

1.00
0.90 (0.72, 1.13)

1.29 (0.91, 1.85)

1.33 (0.93, 1.90)

0.94 (0.77, 1.15)

0.96 (0.77, 1.18)

Deployment experience
Nondeployed
Deployed without combat
exposures
Deployed with combat exposures
Service component
Reserve/Guard
Active duty

1.00

1.00

1.00

1.00

1.19 (0.95, 1.48)

1.20 (0.96, 1.50)

1.28 (1.10, 1.49)

1.35 (1.14, 1.60)

Life stressor scale category
Low/mild

1.00

1.00

1.00

1.00

Moderate

1.12 (0.89, 1.43)

1.18 (0.92, 1.52)

1.15 (0.92, 1.44)

1.15 (0.90, 1.46)

1.24 (0.84, 1.82)

1.20 (0.81, 1.77)

1.75 (1.18, 2.57)

1.77 (1.17, 2.68)

1.00
1.83 (1.45, 2.32)

1.00
1.79 (1.40, 2.30)

1.00
1.88 (1.51, 2.34)

1.00
2.07 (1.61, 2.67)

Major
History of diagnosed mental
disorder
No
Yes
History of alcohol misuse
No

1.00

1.00

1.00

1.00

Yes

1.29 (0.99, 1.68)

1.27 (0.96, 1.68)

1.44 (1.24, 1.67)

1.52 (1.29, 1.79)

Special diet for weight loss
No
Yes (Diet)

1.00

1.00

1.00

1.00

2.26 (1.84, 2.78)

2.27 (1.82, 2.82)

2.54 (2.15, 2.99)

2.42 (2.04, 2.88)

Abbreviations: CI, confidence interval
Details on exclusion criteria and adjustment factors have been published elsewhere [13].

Littman et al. BMC Medical Research Methodology 2010, 10:99
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Table 3 Comparison of associations of new-onset depression based on complete case results (ignoring nonresponse)
and weighted for nonresponse
Adjusted odds ratio (95% CI) of new-onset depression
Women (N = 10,178)

Men (N = 30,041)

Complete case
analysis

Weighted for nonresponse

Complete case
analysis

Weighted for nonresponse

1.00
0.65 (0.47, 0.89)

1.00
0.61 (0.44, 0.84)

1.00
0.66 (0.53, 0.83)

1.00
0.65 (0.52, 0.83)

2.13 (1.70, 2.65)

1.99 (1.58, 2.50)

1.32 (1.13, 1.54)

1.31 (1.12, 1.55)

Deployment experience
Nondeployed
Deployed without combat
exposures
Deployed with combat exposures
Smoking status
Never smoker

1.00

1.00

1.00

1.00

Past smoker

1.30 (1.09, 1.55)

1.34 (1.11, 1.62)

1.18 (1.02, 1.36)

1.23 (1.06, 1.43)

Current smoker

1.35 (1.10, 1.66)

1.42 (1.14, 1.78)

1.52 (1.31, 1.77)

1.57 (1.33, 1.85)

Problem drinking
No
Yes
Posttraumatic stress at baseline

1.00

1.00

1.00

1.00

1.27 (1.03, 1.57)

1.31 (1.04, 1.63)

1.19 (1.04, 1.37)

1.23 (1.06, 1.43)

No

1.00

1.00

1.00

1.00

Yes

2.98 (2.07, 4.28)

3.04 (2.09, 4.43)

4.29 (3.34, 5.50)

3.91 (2.98, 5.13)

Abbreviations: CI, confidence interval
Details on exclusion criteria and adjustment factors have been published elsewhere [25].

limits widened to include 1.0 (OR: 1.60, 95 percent CI:
0.84, 3.04). Similarly, the association between enlisted
rank and new-onset PTSD among Navy and Coast
Guard members was no longer statistically significant
(unweighted OR: 2.14, 95 percent CI: 1.16, 3.94;
weighted for nonresponse OR: 1.99, 95 percent CI: 0.85,
1.68). Nevertheless, in both cases, the point estimates
did not change substantially.
There was little difference in the distribution of selfreported health at follow-up, with and without weighting for nonresponse (data not shown). Adjusting for
nonresponse resulted in a slightly greater proportion of
individuals classified as having poor (0.9 percent
weighted for nonresponse vs. 0.8 percent unweighted),
fair (8.7 percent weighted for nonresponse vs. 8.1 percent unweighted), and good (35.4 percent weighted for
nonresponse vs. 34.5 percent unweighted) health, and a
smaller proportion classified as reporting very good
(38.4 percent weighted for nonresponse vs. 39.2 percent
unweighted) or excellent heath (15.3 percent weighted
for nonresponse vs. 16.1 percent unweighted).

Discussion
Using baseline and follow-up data from the Millennium
Cohort Study, we evaluated nonresponse bias in a large,
relatively young, mobile population of military personnel. A large number of factors were independently associated with response to the follow-up questionnaire. The
characteristics associated with a greater probability of
response included female gender, increasing age, higher

education level, ever married, officer rank, active duty,
and self-reported history of military exposures prior to
2001 (vs. none). Ever smokers, those with a history of
chronic alcohol consumption or a major depressive disorder at baseline, and those who separated from the
military at follow-up, either for retirement or other reason, had a lower probability of response to the followup questionnaire. There was no difference in response
by history of PTSD, panic disorder, or mode of response
(i.e., paper vs. Web) and, thus, these characteristics were
not included in Table 1 or subsequent models. Not surprisingly, many of the factors associated with response
to the follow-up questionnaire were the same ones associated with response to the baseline questionnaire,
namely female gender, older age, non-Hispanic White
or Asian/Pacific Islander race/ethnicity, higher education
level, ever being married, in the Army or Air Force, warrant or commissioned officer, and from health care or
functional support and administration occupations (See
Additional File 1) [18]. Many of these characteristics
(e.g., female gender, older age, and higher education
level) have commonly been associated with greater levels
of response [27-29].
The use of propensity scores and weighting for nonresponse allowed us to determine if previous estimates
made using complete case analysis were affected by nonresponse. Our results indicated that nonresponse did
not substantially affect our estimates of health outcomes
related to deployment or other risk factors. Moreover,
the self-reported general health of cohort members at

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Table 4 Comparison of associations of new-onset posttraumatic stress disorder based on complete case results
(ignoring nonresponse) and weighted for nonresponse
Adjusted odds ratio (95% CI) of new-onset posttraumatic stress disorder
Army (N = 22,958)
Complete
case
analysis

Weighted for
nonresponse

Air Force (N = 14,609)
Complete
case
analysis

Weighted for
nonresponse

Navy and Coast Guard (N =
8,655)
Complete
case
analysis

Weighted for
nonresponse

Marines (N = 2,077)
Complete
case
analysis

Weighted for
nonresponse

Deployment
experience
Nondeployed

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

Deployed
without combat
exposures

0.87 (0.64,
1.18)

0.63 (0.44,
0.92)

0.56 (0.35,
0.89)

0.44 (0.26,
0.77)

0.60 (0.35,
1.02)

0.48 (0.25,
0.90)

1.42 (0.57,
3.51)

0.79 (0.27,
2.36)

Deployed with
combat
exposures

3.59 (3.08,
4.17)

3.55 (3.03,
4.16)

3.38 (2.29,
4.98)

3.61 (2.47,
5.28)

2.48 (1.48,
4.14)

2.27 (1.39,
3.71)

2.78 (1.52,
5.07)

2.87 (1.56,
5.30)

Gender
Male
Female

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.70 (1.44,
2.00)

1.66 (1.39,
1.97)

2.00 (1.41,
2.83)

2.11 (1.48,
3.01)

1.73 (1.25,
2.38)

1.70 (1.19,
2.43)

1.92 (0.94,
3.94)

1.49 (0.65,
3.39)

Smoking
Never smoker

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

Past smoker

1.21 (1.03,
1.44)

1.22 (1.02,
1.46)

1.05 (0.73,
1.50)

0.85 (0.58,
1.25)

1.33 (0.94,
1.88)

1.42 (0.97,
2.07)

1.55 (0.83,
2.88)

1.68 (0.79,
3.58)

Current smoker

1.69 (1.42,
2.01)

1.76 (1.46,
2.11)

1.40 (0.94,
2.07)

1.24 (0.82,
1.89)

1.59 (1.08,
2.34)

2.01 (1.30,
3.10)

1.84 (0.94,
3.59)

2.07 (0.97,
4.41)

Problem drinking
No

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

Yes

1.47 (1.25,
1.73)

1.47 (1.23,
1.74)

1.69 (1.17,
2.43)

1.78 (1.20,
2.64)

1.69 (1.23,
2.34)

1.74 (1.23,
2.46)

1.73 (1.00,
2.99)

1.60 (0.84,
3.04)

2.20 (1.70,
2.86)
1.00

2.31 (1.75,
3.05)
1.00

2.31 (1.24,
4.30)
1.00

2.89 (1.67,
5.00)
1.00

2.14 (1.16,
3.94)
1.00

1.99 (0.85,
1.68)
1.00

1.92 (0.52,
7.13)
1.00

1.56 (0.41,
5.93)
1.00

Military rank
Enlisted
Officer

Abbreviations: CI, confidence interval
Details on exclusion criteria and adjustment factors have been published elsewhere [26].

follow-up did not appear to be different after weighting
for nonresponse. Only where the precision of estimates
was low (e.g., new-onset PTSD among Marines) was
there a meaningful change in the point estimates that
would affect interpretation. Nevertheless, even in this
example, since the results from both the unweighted
and the weighted analyses were imprecise, it would be
imprudent to draw specific conclusions using either
method.
There are several limitations that should be considered
when interpreting our results. First, since we were unable
to collect self-reported follow-up data on nonresponders,
we weighted responses/outcomes among responders
based on a large number of characteristics to reflect
responses of nonresponders at follow-up. Also, we
assumed that the data were MAR and if this assumption
was invalid, we may not have been successful in adjusting
for nonresponse. However, the fact that so much data

were collected at baseline reduces the likelihood that
some unmeasured factors that are associated with nonresponse were not captured [23]. Second, we were unable
to determine whether people did not respond due to
refusal (i.e., they received the questionnaire, but chose
not to complete it) or inability to be contacted (e.g., the
questionnaire was never received due to a change in
address, deployment or occupational situation prevented
contact via postal or electronic mail, or blocked e-mails).
With such a high rate of operational tempo over the past
decade, maintaining contact with military personnel who
deploy frequently and for sometimes lengthy periods of
time or who are required to move and backfill positions
around the country has been a challenge. It is plausible
that frequent residential moves (typical of the current
military lifestyle) may be unrelated to health outcomes.
Nevertheless, we were unable to determine whether nonresponse was associated with outcomes under study and

Littman et al. BMC Medical Research Methodology 2010, 10:99
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potentially incompatible with the MAR assumption. To
obtain a better understanding of the possible reasons for
nonresponse and characteristics of nonresponders, in
2005, a study of 3,000 nonresponders was conducted
(RTI International, unpublished manuscript). Ultimately,
30 percent (n = 908) of the nonresponders were contacted and agreed to answer questions. Self-reported
health status appeared to differ somewhat between
responders to the ancillary study (but nonresponders to
the survey in the current study) and responders. Ancillary
study responders ("nonresponders”) were more likely to
report excellent (25 percent vs. 16 percent weighted for
survey nonresponse) and fair (10 percent vs. 8 percent
weighted for survey nonresponse) or poor (2.1 percent
vs. 0.8 percent weighted for survey nonresponse) health
compared with responders. However, these data are difficult to interpret since those agreeing to participate in the
ancillary study are not likely to be a representative, random sample of all study nonresponders, and instead may
represent a subset of individuals who were too busy
(reflected in the greater proportion reporting excellent
health) or too sick (reflected in the greater proportion
reporting poor health) to respond to the initial survey. A
third potential limitation was our ability to adequately
model response. If response was not adequately modeled,
our ability to adjust for nonresponse would be diminished. However, this seems unlikely since to create the
propensity score, we evaluated a large number of characteristics among baseline responders, including demographic, military, behavioral, and medical characteristics
and the C statistic of 0.71 indicated a good fit of the
model.

Conclusions
In summary, we found that in this relatively young
adult, highly mobile cohort, several factors previously
identified (e.g., male gender, younger age, lower education), as well as some novel factors (e.g., separation
from the military), were associated with lower probability of response. To reduce nonresponse in future followup surveys, it will be important to put additional efforts
into maintaining contact and encouraging participation
for individuals with these characteristics. Furthermore,
because individuals who separate from the military (or
analogously for an occupational cohort, individuals who
are no longer working in the industry) may incorrectly
assume that they are no longer enrolled in the study, it
will be helpful to continue to employ strategies for the
future follow-up cycles that proactively inform these
individuals regarding the importance of their continued
participation, regardless of their current occupational
status. In this study population, nonresponse to the follow-up questionnaire did not result in appreciable bias
as reflected by comparing measures of association for

Page 10 of 11

selected outcomes using complete case and inverse
probability weighted methods. The potential for bias
seemed greatest in subsamples with smaller numbers, as
there were slight differences in point estimates and precision obtained from these two methods of analysis.
Nevertheless, there is no substitute for adequate followup to support proper epidemiologic inference; efforts to
achieve and maintain high response rates are a worthwhile investment in this, and all prospective cohort
studies.

Additional material
Additional file 1: Distribution of various demographic and military
characteristics in the sample invited to participate in the
Millennium Cohort Study in 2001 and responders to the first
follow-up survey in 2004.

Abbreviations
AIC: Akaike’s Information Criterion; CI: confidence interval; MAR, missing at
random; OR: odds ratio; PTSD: posttraumatic stress disorder
Acknowledgements
This material is based upon work supported in part by the U.S. Department
of Veterans Affairs, Office of Research and Development, Cooperative Studies
Program. This work represents report 09.34, supported by the Department of
Defense, under work unit no. 60002. The views expressed in this article are
those of the authors and do not necessarily reflect the position or policy of
the Department of Veterans Affairs, Department of the Navy, Department of
the Army, Department of the Air Force, Department of Defense, or the US
Government. This research has been conducted in compliance with all
applicable federal regulations governing the protection of human subjects
in research (Protocol NHRC.2000.007).
We are indebted to the Millennium Cohort Study participants, without
whom these analyses would not be possible. In addition to the authors, the
Millennium Cohort Study Team includes Gregory C. Gray, MD, MPH, from the
College of Public Health, University of Iowa, Iowa City, IA; James R. Riddle,
DVM, MPH, from the Air Force Research Laboratory, Wright-Patterson Air
Force Base, OH; Margaret A. K. Ryan, MD, MPH, from Naval Hospital Camp
Pendleton, Occupational Health Department; Melissa Bagnell, MPH; Lacy
Farnell; Gia Gumbs, MPH; Nisara Granado, MPH, PhD; Kelly Jones; Cynthia
LeardMann, MPH; Travis Leleu; Jamie McGrew; Amanda Pietrucha, MPH;
Teresa Powell, MS; Donald Sandweiss, MD; Amber Seelig, MPH; Katherine
Snell; Steven Speigle; Kari Welch, MA; Martin White, MPH; James Whitmer;
and Charlene Wong, MPH; from the Department of Deployment Health
Research, Naval Health Research Center, San Diego, CA.
We thank Scott L. Seggerman and Greg D. Boyd from the Management
Information Division, Defense Manpower Data Center, Seaside, CA.
Additionally we thank Michelle Stoia from the Naval Health Research Center.
We also thank all the professionals from the US Army Medical Research and
Materiel Command, especially those from the Military Operational Medicine
Research Program, Fort Detrick, MD. We appreciate the support of the Henry
M. Jackson Foundation for the Advancement of Military Medicine, Rockville,
MD.
Author details
1
Seattle Epidemiologic Research and Information Center, Department of
Veterans Affairs Puget Sound Health Care System, Seattle, WA, USA.
2
Department of Epidemiology, University of Washington, Seattle, WA, USA.
3
Department of Deployment Health Research, Naval Health Research Center,
San Diego, CA. 4Analytic Services, Inc. (ANSER), Arlington, VA, USA.
5
Departments of Preventive Medicine and Biometrics, Uniformed Services
University of Health Sciences, Bethesda, MD, USA. 6Madigan Army Medical
Center, Fort Lewis, WA, USA.

Littman et al. BMC Medical Research Methodology 2010, 10:99
http://www.biomedcentral.com/1471-2288/10/99

Authors’ contributions
AJL advised on the approach for statistical analyses and drafted the
manuscript. EJB conceived the idea for the study and advised on the
approach for statistical analyses. JH and IGJ performed the statistical
analyses. All authors participated in the design of the study, edited and
revised several drafts, and read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 16 March 2010 Accepted: 21 October 2010
Published: 21 October 2010
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Cite this article as: Littman et al.: Assessing nonresponse bias at followup in a large prospective cohort of relatively young and mobile military
service members. BMC Medical Research Methodology 2010 10:99.

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