Combining conditional and unconditional recruitment incentives could
facilitate telephone tracing in surveys of postpartum women
Hind Beydouna, Audrey F. Saftlasa,*, Kari Harlanda, Elizabeth Tricheb
aDepartment of Epidemiology, College of Public Health, University of Iowa, GH C21-F, 200 Hawkins Drive, Iowa City, IA 52242, USA
bYale Center for Perinatal, Pediatric & Environmental Epidemiology One Church St, 6th Floor, New Haven, CT 06510, USA
Accepted 15 November 2005
Background and Objective: To compare tracing and contact rates using alternative incentives in a computer-assisted telephone inter-
view (CATI) survey among postpartum women.
Methods: In a randomized trial of 1,061 postpartum women 18–49 years of age selected from four Iowa counties, we compared the
effects of: (1) unconditional $5 telephone card incentive enclosed with the introductory letter followed by $25 incentive conditional upon
successful telephone tracing, contact, and completion of CATI (Group 1, n 5 530) vs. (2) $30 incentive conditional upon subject comple-
tion of CATI (Group 2, n 5 531).
Results: Overall telephone tracing and contact rates achieved were 67.8% and 66.6%, respectively. Tracing (70.2 vs. 65.4%, P 5.09)
and contact (68.5 vs. 64.8%, P 5.26) rates were consistently higher among subjects assigned the combination of a conditional and an un-
conditional incentive. The combined incentive type had a greater impact on telephone tracing success rates for subjects on whom we could
not initially locate an active telephone number (16.7 vs. 7.3%, P 5.07) when compared to subjects for whom we found an active telephone
number at the time of mailing the introductory letter (78.9 vs. 75.9%, P 5 .30).
Conclusions: Combining conditional and unconditional recruitment incentives can facilitate telephone tracing efforts in surveys
conducted among recently postpartum women. ? 2006 Elsevier Inc. All rights reserved.
Keywords: Computer-Assisted Telephone Interview; Contact rate; Incentive; Postpartum; Tracing rate; Survey
Recent studies have emphasized trends of decreasing
tracing and contact rates in face-to-face interviews , tele-
phone and computer-assisted telephone interviews (CATI)
[2–11] as well as mail [10,12–24] and internet  surveys.
Survey recruitment has become so problematic that a Sum-
mit, sponsored by the U.S. Census Bureau, was held in
2002. In their final report, the Summit Working Group rec-
ommended experimental assessment of unconditional in-
centives to motivate survey recruitment .
Most surveys to date have been conducted among sub-
jects from the general population [1–4,7–10,18,20–22],
while others have targeted low-income people , stu-
dents , older women , and health professionals
[6,12–15,17,19,23]. Few studies have addressed telephone
tracing and contact rates among women of reproductive
age or tested the effectiveness of using different incentives
to promote recruitment in CATI surveys . Survey topics
targeted at women of reproductive age or postpartum
women are often of sensitive nature. In addition, women
who have recently delivered a new infant are less likely
to commit time to a research study due to competing re-
sponsibilities. Thus, we expect that results from previous
studies may not be applicable to this subgroup of women.
Different methods have been used to enhance tracing
and contact rates. Previous research has assessed monetary
18,20,21,24,25] incentives or inducements. Some studies
have compared a monetary incentive to no incentive
[15,19]. Others have evaluated quantitatively different mon-
etary incentives [6,12,16,20,23]. Still others have contrasted
monetary and nonmonetary incentives [17,18,21,24,25],
conditional and nonconditional incentives , as well as
the impact of multiple incentives through a factorial design
The aim of the present article is to compare tracing and
contact rates achieved by two different incentive protocols
* Corresponding author: Tel.: 319-384-5013; fax: 319-384-5004.
E-mail address: Audreyfirstname.lastname@example.org (A.F. Saftlas).
0895-4356/06/$ – see front matter ? 2006 Elsevier Inc. All rights reserved.
Journal of Clinical Epidemiology 59 (2006) 732–738
among postpartum women. Specifically, the investigators
designed this study to determine if tracing and contact rates
could be improved by combining an unconditional incen-
tive with a standard conditional incentive already in use,
without altering the incentive-associated cost per com-
pleted interview. The purpose of an unconditional incentive
is to increase the yield of women who would initiate con-
tact with the project coordinating center, irrespective of
whether or not they had a telephone. Study subjects were
identified from birth certificate records belonging to resi-
dents of four Iowa counties, and invited to participate in
an epidemiologic CATI survey. We hypothesized that
women receiving an unconditional $5 telephone card en-
closed with their introductory letter followed by $25 check
conditional upon CATI completion (Group 1) would
achieve improved rates over women randomized to receive
$30 conditional upon CATI completion (Group 2).
2. Materials and methods
2.1. Study population
A randomized trial of recruitment incentives was de-
signed using a subsample of subjects from a large popula-
tion-based case–control study. Data collection for the latter
study is currently in the final stages. Women of reproduc-
tive age (18–49 years) who resided in four Iowa counties
and delivered a singleton live birth between August and De-
cember 2002 were selected for recruitment in the current
pilot study. Exclusion criteria included women less than
18 years at the time of delivery, multiple births, and those
with diabetes mellitus, systemic lupus erythromatosus, or
chronic renal disease.
2.2. Field operations
Approval from the Institutional Review Board at the
University of Iowa was received to conduct this methodo-
logic randomized pilot study. A uniform protocol of field
operations was implemented for all subjects selected for
tracing. Initially, home addresses were obtained from birth
certificate records and telephone numbers were traced using
Web-based telephone directories, reverse directories, and
directory assistance. Once a telephone number was ascer-
tained, a ‘‘standard’’ introductory letter was sent. Other-
wise, a ‘‘no-telephone’’ introductory letter was sent,
which asked subjects to mail their telephone information
to the research office or to contact the project’s toll-free
number. If the introductory letter was returned by the post
office with a forwarding address, the new address was used
for retracing the telephone number and another letter was
sent according to the protocol. Therefore, some of the sub-
jects who initially received a standard introductory letter
could end up with the disposition ‘‘untraceable’’ if an active
telephone number is not found or provided by the subject.
Other subjects who initially received a ‘‘no-telephone’’
letter may have an active telephone number identified by
the end of the study period. It is worth noting that efforts
were focused on identifying land line or regular telephone
numbers. At the time the study was initiated, tracing could
not be accomplished using cellular phone subscriber lists.
In addition, potential participants were not asked whether
they relied solely on a cellular telephone or if they also
had a land line.
Once an active telephone number was obtained, the sub-
ject was contacted by a project interviewer to: (1) obtain
verbal consent for CATI participation and agreement to
sign a medical record release form, (2) verify eligibility us-
ing a brief 3-min screening interview, and (3) set an ap-
pointment for the CATI. Telephone numbers were called
at times that included mornings, afternoons, and evenings
on weekdays. If the interviewer was not able to reach the
subject within six call attempts, an ‘‘unable-to-contact’’ let-
ter was sent and a maximum of four additional calls were
attempted. Within the six call attempts, the interviewer
was instructed to provide information to potential subjects
about the study directly or through their answering ma-
chines. The length of the CATI interview averaged 50
min, with a range of 45 to 70 min. Monetary incentives
of either $25 (Group 1) or $30 (Group 2) were mailed to
all subjects who completed the CATI interview and agreed
to provide researchers with their signed medical record re-
2.3. Randomized trial of recruitment incentives
A total of 1,061 women were randomly assigned to ei-
ther one of two groups. Radomization was performed
through the computerized database by assigning each re-
cord, at random, to two nearly equal sized groups. Group
1 (n 5 530) received an unconditional $5 telephone card in-
cluded with the introductory letter, followed by a $25 check
provided conditional upon completion of the CATI. Group
2 (n 5 531) was offered a $30 check conditional upon com-
pletion of the CATI. Prior to the trial, we had provided
a $30 conditional incentive but were not meeting our pro-
jected tracing rate. Blinding of interviewers with respect
to incentive assignment could not be achieved for logistical
reasons. However, technicians who performed telephone
tracing activities were blinded to the subjects’ incentive
group assignment. Aside from the randomly assigned in-
centive type, other field aspects of the trial were equivalent
for the two groups. Baseline data were obtained on all ran-
domized subjects from the birth certificate files and linked
to the detailed contact and call record database, which con-
tains information on incentive assignment and field out-
comes. Final tracing outcomes were: (1) active telephone
traced and (2) no active telephone traced. Final contact out-
comes were coded as one of the following: (1) agreed to be
screened, (2) refused to be screened, and (3) no contact es-
tablished because of inability to locate a telephone number
or to reach selected subject.
733 H. Beydoun et al. / Journal of Clinical Epidemiology 59 (2006) 732–738
2.4. Sample size calculations
The present analyses were conducted on a fixed sample
size of 1,062 postpartum women selected from birth certif-
icate files. Assuming equal allocation of subjects to treat-
ment Groups 1 and 2, there would be 93% power to
detect a 10% change (e.g., an increase in tracing or contact
rate from 65 to 75%) at an alpha level of 0.05.
2.5. Statistical analysis
All statistical analyses were conducted using the SAS
system version 9.0.
The Pearson chi-square test was used to compare Group
1 and Group 2 on baseline characteristics. Tracing rates
were defined as the number of subjects on whom an active
telephone number could be traced by the end of the study
period divided by the total number of potentially enrolled
subjects. Contact rates were calculated using formulas re-
ported by Slattery et al. . Independent samples tests
comparing differences in tracing and contact rates among
randomization groups were conducted using the normal ap-
proximation to the binomial distribution, assuming equal
variances. Statistical significance and borderline statistical
significance were determined at alpha levels of 0.05 and
Table 1 presents baseline characteristics of subjects ran-
domly assigned to either Group 1 or Group 2. As expected,
no statistically significant differences (P ! .05) were ob-
were between 20 and 34 years of age, with a mean age of 27
years at delivery. Nearly 33.3% had completed at least 4
years of college. Over 75% were non-Hispanic Whites and
66% were married. Almost 40% were nulliparous and 23%
had a previous abortion. More than 99% received prenatal
care and 17% had smoked during pregnancy, as recorded
on the birth certificate. Based on data from the Centers for
Disease Control and Prevention , our study sample is
comparable to U.S. women (15–44 years of age) who gave
birth in 2002 on various indicators including the percentage
married or receiving prenatal care. On the other hand, 26%
of all U.S. women had completed at least 4 years of college,
78% were non-Hispanic White and 11.4% smoked during
pregnancy. Such disparities could be partially attributed to
the exclusion of teenage pregnancies less 18 years of age
from our study sample and more complete recording of
smoking on Iowa birth certificates.
In Figure 1, survey outcomes are described by incentive
group and type of introductory letter received. Out of 1,061
potentially enrolled subjects, 900 (84.8%) (Group 1: 85.4%
and Group 2: 84.5%) had a telephone number that could be
initially traced and thereby received a ‘‘standard’’ introduc-
tory letter. On the other hand, 154 (15.2%) (Group 1: 13.6%
and Group 2: 15.5%) of the subjects initially did not have
a traceable telephone number and were therefore sent
a ‘‘no-telephone’’introductory letter. Telephone contact
was established with 618 (58.2%) postpartum women
(Group 1: 60.5% and Group 2: 55.9%). Once contact with
a potential subject was established, more than 80% of po-
tentially eligible women agreed to be screened according
to prespecified eligibility criteria. It is worth noting that
three subjects (two in Group 1 and one in Group 2) were
successfully contacted even though their telephone number
could not be traced. These subjects had taken the initiative
to contact the coordinating center using the 1-800- number
provided in the body of the introductory letter.
Baseline characteristics of potentially enrolled subjects by incentive
(n 5 1,061)
(n 5 530)
(n 5 531)
Secondary or less
College (1–3 years)
College (41 years)
Number of prenatal care visits
Tobacco during pregnancy
135 (25.47) 154 (29.00)
169 (31.89) 144 (27.12)
117 (22.08) 128 (24.11)
64 (12.05) .41
215 (40.57) 223 (42.00)
130 (24.53) 139 (26.18)
185 (34.91) 169 (31.83) .56
410 (77.36) 401 (75.52)
350 (66.04) 349 (65.73)
180 (33.96) 182 (34.27) .91
215 (40.57) 210 (39.62)
315 (59.43) 320 (60.38) .75
406 (76.60) 394 (74.20)
119 (22.45) 132 (24.86)
5 (0.94) 5 (0.94).65
174 (32.83) 184 (34.65)
445 (83.96) 441 (83.05)
bGroup 2 received conditional $30.
cPrevious abortions, before 20 weeks of gestation.
734H. Beydoun et al. / Journal of Clinical Epidemiology 59 (2006) 732–738
A stratified analysis of tracing and contact rates among
the randomization groups by introductory letter type is pre-
sented in Table 2. The overall tracing and contact rates
achieved were 67.8 and 66.6%, respectively. Tracing
(70.2 vs. 65.4%, P 5 .09) and contact (68.5 vs. 64.8%,
P 5.26) rates were consistently higher among subjects ran-
domized to receive a combination of conditional and un-
conditional incentives. The differences in tracing rates
were of borderline significance (P ! .1). Furthermore,
the combined incentive had a substantially improved effect
on tracing rates for subjects who initially did not have an
active telephone number (16.7 vs. 7.3%, P 5 .07), but
had little effect on tracing rates among those who initially
had an active telephone number (78.9% vs. 75.9%, P 5.3).
Despite modest achievements in improving tracing and
contact rates, addition of a telephone card as an uncondi-
tional incentive was instrumental in reaching a subgroup
of socioeconomically disadvantaged women for whom
a telephone number could not be traced initially. Further
analyses confirmed that this subgroup had a greater
aGroup 1 received unconditional $5 telephone card and conditional $25; bGroup 2 received conditional $30;
cIn seven subjects, no data were available on initial type of introductory letter.
Incentive study sample
Standard intro letterc
Traced: 357; Not traced: 95
No phone intro letterc
Traced: 12; Not traced: 60
Standard intro letterc
Traced: 340; Not Traced: 108
No phone intro letterc
Traced: 6; Not traced: 76
Agree to be screened
Agree to be screened
Agree to be screened
Agree to be screened
Fig. 1. Pilot study summary of field operation outcomes.
Tracing and contact rates by incentive group and type of introductory letter
Standard introductory letter
No telephone introductory letter
Abbreviations: T, active telephone traced; NT, no active telephone traced; I, interviewed individuals; P, partially interviewed individuals; R, refusals; NE,
not eligible for study criteria; NCP, no contact because of inability to locate a number or unable to reach selected subject.
aTracing rate 5 T/(T1NT).
bContact rate 5 (I1P1R1NE)/(I1P1R1NCP).
cIn seven subjects, no data was available on initial type of introductory letter.
dGroup 1 received unconditional $5 telephone card and conditional $25.
eGroup 2 received conditional $30.
735H. Beydoun et al. / Journal of Clinical Epidemiology 59 (2006) 732–738
percentage of young, unmarried, less educated, non-White
women who reported a higher smoking rate during preg-
nancy, and had fewer prenatal care visits (Table 3).
Few studies to date have assessed the effectiveness of
different incentives in improving recruitment to a CATI sur-
vey [6,30] among postpartum women, who constitute a dis-
tinct population from health professionals and other special
populations, and might respond differently to similar re-
cruitment strategies . In the current randomized con-
trolled trial, the group of women randomly assigned to
receive an unconditional $5 telephone card in addition to
a $25 conditional incentive were traced and contacted at
higher rates than women who were randomized to $30 con-
ditional incentive. Among women who initially received
a ‘‘no-telephone’’ introductory letter, the $5 telephone card
was particularly instrumental in achieving higher tracing
and contact rates.
Several factors have played a role in determining the
tracing and contact rates in our study. These include char-
acteristics of subjects (postpartum women), survey method
used (CATI), efforts spent on tracing and recruitment of
subjects (field operations), in addition to qualitative and
quantitative characteristics of the incentives provided
(check and telephone card). Often, the survey method is
predetermined by the target population of interest, espe-
cially when subjects are health professionals [6,12–
15,17,19,23]. In other situations, a choice of different sur-
vey methods, including telephone [2–4,8], mailed [16,20],
or direct interview  surveys or a combination of these
[5,10] could be applied. Telephone and CATI surveys have
manyadvantages over other methodsof survey
Baseline characteristics of potentially enrolled subjects by type of introductory letter received initially
Totala(n 5 1,054)
letter (n 5 900)
letter (n 5 154)
Secondary or less
College (1–3 years)
College (41 years)
Number of prenatal care visits
Tobacco during pregnancy
92 (59.74) .98
aIn seven subjects, no data were available on initial type of introductory letter.
bPrevious abortions, before 20 weeks of gestation.
736H. Beydoun et al. / Journal of Clinical Epidemiology 59 (2006) 732–738
administration. However, some major obstacles to achiev-
ing adequate recruitment have been encountered recently
in telephone and CATI surveys. Some issues in telephone
tracing and contact that have become particularly problem-
atic in recent years include increased usage of unlisted cel-
lular telephone numbers, less tolerance for interruptions of
private life by telemarketers, and greater usage of technol-
ogy that screens incoming telephone calls [31,32].
To date, most studies that have assessed the impact of
monetary or nonmonetary incentives on recruitment suc-
cess were limited to mailed surveys of health professionals
[12–15,17,19,23,30]. These studies suggested varying im-
provements in subject recruitment by type of incentive,
with monetary incentives being more successful than other
In the current pilot study of recruitment incentives, mod-
erate overall tracing (67.8%) and contact (66.6%) rates
were achieved, irrespective of inducement strategy. The
telephone card incentive played a modest role in improving
these rates, but was highly instrumental in motivating sub-
jects whose telephone number could not be initially traced.
Findings from the pilot study have been translated into
further changes in the survey protocol, among which was
the administration of the more effective incentive type
(unconditional $5 telephone card enclosed with their in-
troductory letter followed by $25 check conditional upon
CATI completion) to all subsequently identified postpartum
women and implementation of Saturday recruiting and
In conclusion, combining conditional and unconditional
recruitment incentives can facilitate telephone tracing ef-
forts in surveys conducted among postpartum women.
This study was supported by grant RO1-HD39753-01
from the National Institute of Child Health and Human
Development. We would like to thank all tracing and
interviewing staff who worked on this pilot project.
 Pavlik VN, Hyman DJ, Vallbona C, Dunn JK, Louis K, Dewey CM,
Wieck L, Toronjo C. Response rates to random digit dialing for re-
cruiting participants to an onsite health study. Public Health Rep
 Anie KA, Jones PW, Hilton SR, Anderson HR. A computer-assisted
telephone interview technique for assessment of asthma morbidity
and drug use in adult asthma. J Clin Epidemiol 1996;49:653–6.
 Blyth FM, March LM, Shellard D, Cousins MJ. The experience of
using random digit dialing methods in a population-based chronic
pain study. Aust N Z J Public Health 2002;26:511–4.
 Corkrey R, Parkinson L. A comparison of four computer-based tele-
phone interviewing methods: getting answers to sensitive questions.
Behav Res Methods Instrum Comput 2002;34:354–63.
 Groves RM, Mathiowetz NA. Computer assisted telephone interview-
ing: effects on interviewers and respondents. Public Opin Q 1984;48:
 Gunn WJ, Rhodes IN. Physician response rates to a telephone survey:
effects of monetary incentive level. Public Opin Q 1981;45:109–15.
 Hornik J, Zaig T, Shadmon D, Barbash GI. Comparison of three
inducement techniques to improve compliance in a health survey
conducted by telephone. Public Health Rep 1990;105:524–9.
 Ketola E, Klockars M. Computer-assisted telephone interview
(CATI) in primary care. Fam Pract 1999;16:179–83.
 Slade GD, Brennan D, Spencer AJ. Methodological aspects of a
computer-assisted telephone interview survey of oral health. Aust
Dent J 1995;40:306–10.
 Wang PS, Beck AL, McKenas DK, Meneades LM, Pronk NP,
Saylor JS, Simon GE, Walters EE, Kessler RC. Effects of efforts to
increase response rates on a workplace chronic condition screening
survey. Med Care 2002;40:752–60.
 Wilkins D, Casswell S, Barnes HM, Pledger M. A pilot study of
a computer-assisted cell-phone interview (CACI) methodology to
survey respondents in households without telephones about alcohol
use. Drug Alcohol Rev 2003;22:221–5.
 Asch DA, Christakis NA, Ubel PA. Conducting physician mail sur-
veys on a limited budget. A randomized trial comparing $2 bill versus
$5 bill incentives. Med Care 1998;36:95–9.
 Deehan A, Templeton L, Taylor C, Drummond C, Strang J. The effect
of cash and other financial inducements on the responser of general
practitioners in a national postal study. Br J Gen Pract 1997;47:
 Delnevo CD, Abatemarco DJ, Steinberg MB. Physician response
rates to a mail survey by specialty and timing of incentive. Am J Prev
 Everett SA, Price JH, Bedell AW, Telljohann SK. The effect of a
monetary incentive in increasing the return rate of a survey to family
physicians. Eval Health Prof 1997;20:207–14.
 Gibson PJ, Keopsell TD, Diehr P, Hale C. Increasing response rates
for mailed surveys of Medicaid clients and other low-income popula-
tions. Am J Epidemiol 1999;149:1057–62.
 Oden L, Price JH. Effects of a small monetary incentive and follow-
up mailings on rates of a survey to nurse practitioners. Psychol Rep
 Perneger TV, Etter JF, Rougemont A. Randomized trial of use of
a monetary incentive and a reminder to increase the response rate
to a mailed health survey. Am J Epidemiol 1993;138:714–22.
 Russell ML, Mustasingwa DR, Verhoef MJ, Injeyan HS. Effect of
monetary incentive on chiropractors’ response rate and time to re-
spond to a mail survey. J Clin Epidemiol 2003;56:1027–8.
 Shaw MJ, Beebe TJ, Jensen HL, Adlis SA. The use of monetary in-
centives in a community survey: impact on response rates, data qual-
ity, and cost. Health Serv Res 2001;35:1339–46.
 Spry VM, Hovell MF, Sallis JG, Hofsteter CR, Elder JP,
Molgaard CA. Recruiting survey respondents to mailed surveys:
controlled trials of incentives and prompts. Am J Epidemiol 1989;
 Stockbridge H, Hardy RI, Glueck CJ. Public cholesterol screening:
motivation for participation, follow-up outcome, self-knowledge,
and coronary heart disease risk factor intervention. J Lab Clin Med
 VanGeest JB, Wynia MK, Cummins DS, Wilson IB. Effects of differ-
ent monetary incentives on the return rate of a national mail survey of
physicians. Med Care 2001;39:197–201.
 Whiteman MK, Langenberg P, Kjerulff K, McCarter R, Flaws JA.
A randomized trial of incentives to improve response rates to a mailed
women’s health questionnaire. J Womens Health 2003;12:821–8.
 Kypri K, Gallaguer SJ. Incentives to increase participation in an
internet survey of alcohol use: a controlled experiment. Alcohol
 Salvucci S, Wenck S, Hamsher S, Bates N. Response rate sum-
mitdNational Health Interview & Consumer Expenditure Quarterly
Surveys. U.S. Census Bureau and the Interagency Household Survey
Nonresponse Group; 2002.
737H. Beydoun et al. / Journal of Clinical Epidemiology 59 (2006) 732–738
 Brown JE, Jacobs DR Jr, Barosso GM, Potter JD, Hannan PJ, Download full-text
Kopher RA, Rourke MJ, Hartman TJ, Hase K. Recruitment, retention
and characteristics of women in a prospective study of preconcep-
tional risks of reproductive outcomes: experience of the Diana Pro-
ject. Paediatr Perinat Epidemiol 1997;11:345–58.
 Slattery ML, Edwards SL, Caan BJ, Kerber RA, Potter JD. Response
rates among control subjects in case–control studies. Ann Epidemiol
 Centers for Disease Control and Prevention. National vital statistics
reports. Births: final data for 2002. December 17, 2003.
 Edwards P, Cooper C, Roberts R, Frost C. Meta-analysis of random-
ized trials of monetary incentives and response to mailed question-
naires. J Epidemiol Community Health 2005;59:987–99.
 Nayak MB, Kaskutas LA. Risky drinking and alcohol use patterns in
a national sample of women of childbearing age. Addiction 2004;99:
 Midanik LT, Greenfield TK. Defining ‘‘current drinkers’’ in national
surveys: results of the 2000 National Alcohol Survey. Addition
738 H. Beydoun et al. / Journal of Clinical Epidemiology 59 (2006) 732–738