Randomized Trial of Financial Incentives and Delivery Methods for Improving Response to a Mailed Questionnaire

Michele Morin Doody1,, Alice S. Sigurdson1, Diane Kampa2, Kathleen Chimes3, Bruce H. Alexander2, Elaine Ron1, Robert E. Tarone1 and Martha S. Linet1

1 Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Bethesda, MD.
2 Division of Environmental and Occupational Health, University of Minnesota, Minneapolis, MN.
3 Westat, Incorporated, Rockville, MD.

Received for publication December 14, 2001; accepted for publication October 28, 2002.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In a follow-up study, only 64% of 126,628 US radiologic technologists completed a questionnaire during 1994–1997 after two mailings. The authors conducted a randomized trial of financial incentives and delivery methods to identify the least costly approach for increasing overall participation. They randomly selected nine samples of 300 nonresponders each to receive combinations of no, $1.00, $2.00, and $5.00 cash or check incentives delivered by first-class mail or Federal Express. Federal Express delivery did not achieve greater participation than first-class mail (23.2% vs. 23.7%). In analyses pooled across delivery methods, the response was significantly greater for the $2.00 bill (28.9%, 95% confidence interval (CI): 25.2, 32.7; p < 0.0001), $5.00 check (27.5%, 95% CI: 22.5, 33.0; p = 0.0001), $1.00 bill (24.6%, 95% CI: 21.2, 28.3; p = 0.0007), and $2.00 check (21.8%, 95% CI: 18.5, 25.3; p = 0.02) compared with no incentive (16.6%, 95% CI: 13.7, 19.9). The response increased significantly with increasing incentive amounts from $0.00 to $2.00 cash (p trend < 0.0001). The $2.00 bill achieved a 30% greater response than did a $2.00 check (p = 0.005). For incentives sent by first-class mail, the $5.00 check yielded 30% greater participation than did the $2.00 check (p = 0.07). A $1.00 bill, chosen instead of the $2.00 bill because of substantially lower overall cost and sent by first-class mail to the remaining 42,717 nonresponders, increased response from 64% to 72%.

cohort studies; data collection; epidemiologic methods; motivation; nonresponse; postal service; questionnaires; randomized controlled trials

Abbreviations: Abbreviation: CI, confidence interval.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Postal questionnaires are often the only economically feasible alternative for obtaining information in epidemiologic studies (1). Response to mailed questionnaires varies according to population characteristics, the number of times subjects are approached, the content and wording of questions, and questionnaire length (2). Low response rates can threaten study validity (38) by introducing a selection bias if nonresponse is disproportionate. A small monetary incentive has been shown to significantly improve response (922). To improve response among nonresponders of a questionnaire mailed to a nationwide cohort at lowest cost, we conducted a randomized trial to compare simultaneously nine different combinations of incentive amount, type, and delivery method.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In 1982, the National Cancer Institute and the University of Minnesota initiated a cohort investigation to estimate cancer risk among approximately 146,000 radiologic technologists first certified by the American Registry of Radiologic Technologists between 1926 and 1982. The study flow chart is shown in figure 1. A structured questionnaire was sent during 1984–1987 by US first-class mail; 90,305 of 132,454 (68 percent) technologists who were presumed to be alive because of their having recertified with the American Registry of Radiologic Technologists within the previous year or having failed to link with national death record databases responded, providing information on prior cancers and selected other diseases, lifetime work history as a radiologic technologist, procedures performed, equipment used, and potential confounding factors (23). In a second follow-up, mailed questionnaires were sent during 1995–1997 to all 126,628 subjects who were presumed alive. We again inquired about all physician-diagnosed cancers, other medical conditions, employment, and cancer risk factors. By 1997, 64 percent had responded to the second survey following two questionnaire-mailing waves. We initiated a telephone follow-up of the 45,576 nonresponders; however, because of the magnitude of this effort, we later focused on the subgroup of 19,807 technologists who had responded to the first survey. We contacted 31 percent of all nonresponders; for 15 percent interviewers spoke directly with the subject, for 10 percent they left messages with family members, and for 6 percent they left messages on answering machines. The low participation led us to initiate a randomized trial to identify a cost-effective method for improving questionnaire completion among nonresponders. The randomized trial of financial incentives was approved by the institutional review boards at the National Cancer Institute and the University of Minnesota.



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FIGURE 1. Diagram of the target population of radiologic technologists as of the first and second survey mailings and the number of questionnaire responses, US Radiologic Technologists (USRT) Study, 1983–1998. The USRT cohort was initially established in 1982 and included 143,517 technologists who were certified by the American Registry of Radiologic Technologists for at least 2 years during 1926–1982. An additional 2,505 eligible technologists were later identified and added, bringing the total cohort to 146,022 technologists. As of the first survey, 6,350 technologists were deceased; as of the second survey, an additional 5,377 technologists were deceased, including 3,104 who completed the first survey.

 
We randomly selected nine samples with 300 nonresponders each and then updated vital status and contact information. A few subjects in each group were deceased, could not be located, or responded to the questionnaire after they were selected but before the trial began; these subjects were excluded. There were no statistically significant differences among the nine groups in demographic characteristics and the number of years certified as a radiologic technologist (table 1). Telephone contact rates did not differ significantly among incentive groups (data not shown). We compared financial incentives of $1.00 cash, $2.00 cash or check, and $5.00 check, as well as delivery by US first-class mail or Federal Express (Memphis, Tennessee). We did not test a $1.00 check, which might be perceived as a nuisance, nor did we test a $5.00 cash incentive or check sent by Federal Express, because these were not economically feasible incentives. Thus, compared with a referent group who received a questionnaire sent by US first-class mail with no financial incentive, eight different combinations of incentive type, amount, and method of delivery were evaluated (table 2).


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TABLE 1. Characteristics of radiologic technologists included in a randomized trial of financial incentives and delivery methods for improving questionnaire response, by incentive group, US Radiologic Technologists Study, 1997–1998*
 

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TABLE 2. Results of randomized trial of financial incentives and delivery methods for improving questionnaire response among radiologic technologists, US Radiologic Technologists Study, 1997–1998
 
All randomized nonresponders were included in the analysis whether they had a street address or a post office box (i.e., analysis was by intent to treat). Subjects who were assigned to receive a questionnaire by first-class mail were prompted by a prenotification letter because the questionnaire was mailed in the same color and size of envelope that was used in the two previous mailings. No advance letter was sent if questionnaires were sent by Federal Express. Regardless of the delivery method, we included a postscript on the questionnaire’s cover letter to draw attention to the enclosed monetary incentive and a letter supporting the study from the American Society of Radiologic Technologists. Questionnaires returned by the postal service or Federal Express with forwarding addresses were remailed; subjects with post office addresses that could not be delivered by Federal Express were counted as nonresponders. The percentages of questionnaires that were undeliverable ranged from 1.0 percent to 3.0 percent for incentives sent by first-class mail and from 2.0 percent to 6.4 percent for incentives sent by Federal Express.

We used chi-square tests to evaluate the overall differences in response of the various groups compared with the referent group and to evaluate the demographic differences in response within incentive groups (e.g., men vs. women among those who were sent a $1.00 bill) and among demographic categories between incentive groups (e.g., women who received a $1.00 bill vs. women who received no incentive). We also evaluated check-cashing behavior according to the amount ($2.00, $5.00) of the check and the questionnaire response (no, yes). The Bonferroni method was used to assess possible chance findings associated with multiple comparisons (i.e., by multiplying each p value of <0.05 by the number of different incentives).

We calculated the costs per mailing and response for the incentive groups. We used the incentive trial findings to project the response rates and costs for a final mailing to the 43,000 remaining nonresponders, and we present response rates for the final mailing.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Features of the 126,628 presumed living technologists targeted for the second survey were generally similar to those of the subset of 45,576 nonresponders to two questionnaire mailings, including attained age, geographic residence, and number of years certified as a radiologic technologist (data not shown). Male technologists were disproportionately represented among nonresponders (31 percent) compared with the second survey-eligible group (21 percent), as were baseline survey nonresponders (57 percent vs. 33 percent).

Compared with that of the referent group (response, 15.5 percent), response rates were 10–90 percent higher among the various incentive-delivery groups (table 2). Statistically significant improvements in response were seen for all cash incentives, regardless of the delivery method, and for the $5.00 check sent by first-class mail and the $2.00 check sent by Federal Express; Federal Express delivery without any incentive and a $2.00 check sent by first-class mail did not significantly improve response. Because Federal Express delivery did not improve response rates over comparable incentives sent by first-class mail (23.2 percent vs. 23.7 percent, respectively), we pooled the results by delivery method. As shown in table 3, compared with no incentive (16.6 percent response, 95 percent confidence interval (CI): 13.7, 19.9), significant improvements in response were seen with all of the pooled amount/type incentives. The $2.00 bill yielded the best response (28.9 percent, 95 percent CI: 25.2, 32.7; p < 0.0001), followed by the $5.00 check (27.5 percent, 95 percent CI: 22.5, 33.0; p = 0.0001), $1.00 bill (24.6 percent, 95 percent CI: 21.2, 28.3; p = 0.0007), and $2.00 check (21.8 percent, 95 percent CI: 18.5, 25.3; p = 0.02). The association for the $2.00 check became nonsignificant (p = 0.10), however, following adjustment for multiple comparisons. Only completed questionnaires were counted as a response.


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TABLE 3. Number of radiologic technologists included in the financial incentive trial and the percentage who responded, by personal characteristics and amount and type of financial incentive, US Radiologic Technologists Study, 1997–1998
 
The response increased significantly with increasing incentive amounts from $0.00 to $2.00 cash (p trend < 0.0001). The $2.00 bill achieved a statistically significant 30 percent greater response than a $2.00 check ({chi}2 = 7.81; p = 0.005). For incentives sent by first-class mail, there was a 30 percent improvement for the $5.00 check compared with the $2.00 check ({chi}2 = 3.21; p = 0.07).

Responses by demographic characteristics (i.e., within incentive group comparisons) are shown in table 3. Men and women responded similarly to all incentives, except for a 50 percent greater response by women to the $5.00 check. The number of non-White persons was small, but statistically significant differences were apparent by race within all incentive groups; Blacks typically responded at a much lower rate than did Whites, and persons in the "other" racial category responded especially poorly. There were no statistically significant differences in response by age, although technologists who were 60–69 years of age responded especially well to the $5.00 check (52.0 percent). Responses to the various incentives did not vary significantly among the four major US Census-defined geographic regions of residence (North, South, Midwest, West); however, among the nine smaller Census-defined geographic divisions, especially low response rates were seen within the no incentive group in the Middle Atlantic (10.0 percent), East North Central (5.3 percent), and West South Central (10.4 percent) divisions (data not shown). The number of years a technologist was certified did not significantly influence the response to any of the incentives. Persons participating in the first survey were significantly more likely to respond to the second survey (ranging from 40 percent ($5.00 check) to 2.4 times ($2.00 check) higher than those who did not respond to the first survey).

Between-group differences (i.e., response to specific incentives vs. no incentive) were also seen according to demographic characteristics (table 3). Compared with their counterparts who received no incentive, men who received a $1.00 or $2.00 bill and women who received a $2.00 bill or $5.00 check were significantly more likely to respond. Significant improvements in response were seen for Whites with all but the $2.00 check and for persons in the "other" racial category with the $2.00 bill. The small number of Black respondents made differences in response across incentives difficult to interpret. The $2.00 bill afforded substantial and, in most cases, statistically significant improvements in response among all technologists 40 or more years of age and improved response regardless of the number of years the technologist was certified. Significant improvements in response were seen for the $2.00 bill in the Midwest and South Census geographic regions and for the $2.00 check in the Midwest. The $1.00 bill was effective in all but the West region, while the $5.00 check improved response in all regions; note, however, that these associations were not statistically significant after adjusting for multiple comparisons. Although based on small numbers, residents of the East South Central Census division, which encompasses much of Appalachia, responded very favorably to cash incentives of $1.00 (41.9 percent; n = 14) and $2.00 (45.2 percent; n = 14) (data not shown). All incentives significantly improved response among the first survey responders, although the association with the $5.00 check was not significant after adjustment for multiple comparisons; however, only the $2.00 bill and the $5.00 check prompted significant increases among nonresponders to the first survey.

Among individuals who were sent checks, in general, 89.6 percent of those who responded to the questionnaire cashed their checks, while only 9.4 percent of those who did not respond cashed their checks (table 4). Both responders and nonresponders were more likely to cash the $5.00 check than the $2.00 check. Within groups defined by the second survey response and demographic characteristics, there were no significant differences in check-cashing behavior (data not shown).


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TABLE 4. Check-cashing behavior among radiologic technologists according to questionnaire response status and amount of check, US Radiologic Technologists Health Study, 1997–1998
 
For questionnaires sent by first-class mail, the cost per questionnaire returned was lowest for the $2.00 bill ($25.09) and highest for the $2.00 check ($29.65) (table 2). The cost per response for the $1.00 bill ($25.50) was similar to that for the $2.00 bill. Use of Federal Express delivery added costs of about $8.50 per response.

Use of any of the incentive amount/type/delivery combinations with the remaining 43,000 nonresponders was projected to achieve an overall response greater than 70 percent among those eligible for the second survey. We opted to send a final questionnaire by first-class mail with a $1.00 cash incentive. This method was the second most efficient in cost per response and had the next to lowest overall cost (only the $2.00 check was lower). We sent letters in advance to notify the prior nonresponders that a small token of our appreciation would be included with a forthcoming questionnaire. We received 9,303 additional responses, bringing the final response rates to 72 percent overall and 83 percent among first survey responders. The response to the final mailing was similar for men and women, higher among Whites, and higher among first survey responders; the response was lower among technologists who were 70 or more years of age and among those certified for less than 10 years (data not shown). There was little difference in response by geographic residence.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Our randomized trial revealed that a small monetary incentive was effective in converting a number of reluctant responders. Cash yielded a significantly better response than did checks, and the response increased significantly with increasing incentive amounts ($0.00, $1.00, $2.00). The $2.00 bill, perhaps because of its novelty, achieved the best response. We found no advantage to using the more expensive Federal Express delivery method over first-class mail.

Compared with previous randomized trials of financial incentives (1, 1022, 2426) and methods of questionnaire delivery (12, 18, 22, 27, 28), our study had several unique features. We simultaneously compared nine combinations of incentive amount and type, as well as delivery methods, whereas previous studies generally compared from two to six approaches. This is one of the first studies to directly compare responses to cash vs. check incentives of the same amount, to evaluate check-cashing behavior by the amount of the check, and to assess geographic differences in the response and cost per questionnaire returned by incentive amount and type. Finally, we describe the final outcome among the remaining 43,000 nonresponders in this nationally distributed US cohort.

Strategies shown to improve response in previous studies include the use of first-class mail (10, 12), certified mail with a return receipt (12, 18, 27, 28), stamped envelopes (12), personalized mail-out packages (27, 29), short questionnaires (12), monetary incentives from $1.00 to $50.00 (12, 13, 17, 18, 20, 3038), prenotification and follow-up mailings (10), colored questionnaires (especially green) (10), university sponsorship (10), lotteries (19, 39, 40), Federal Express delivery (22), and commemorative stamps (41). Prepayments of financial incentives have generally resulted in better responses than promises of postpayment (11, 13, 16, 42). A systematic review of the effectiveness of these methods was recently published (43). Caution in the use of incentives may be warranted in studies with repeated surveys; one study reported that the response to a second survey sent without an incentive was lower for subjects who received an incentive with the initial questionnaire than for those who did not (34).

Our findings for small cash or check incentives ($1.00, $2.00, $5.00) resemble the results reported by others (13, 17, 18, 20, 3038), although a higher response to cash than checks has not been previously noted. We identified only one other study that compared Federal Express with US first-class mail delivery (22); the authors reported a significantly greater response among a small group of physicians sent questionnaires by Federal Express. The lack of improvement with Federal Express observed in our study may reflect the use of prenotification letters with questionnaires sent by first-class mail but not with questionnaires sent by Federal Express.

Consistent with previous reports (1416, 18, 21, 26, 32), we found little difference in response by gender or attained age. Although the numbers of non-Whites were small in each incentive group, the significant difference in response by race within our nationally distributed cohort contrasts with a lack of difference among cosmetologists in North Carolina (15). The higher response by first survey responders compared with nonresponders is consistent with a previous study (16), although our finding may reflect a more intensive prior effort to telephone first survey responders. We did not find other US reports evaluating various incentives in relation to geographic residence.

The overall response (21.8 percent) of the 43,000 nonresponders to the final mailing of a questionnaire with a $1.00 bill was lower than projected for the $1.00 bill in the incentive trial (24.6 percent). Demographic patterns in response to the final mailing were not inconsistent with those in the incentive trial, although less variation was seen across geographic divisions than was observed in the much smaller incentive trial sample.

The most effective incentive was the $2.00 bill. The "novelty" of the bill, rather than the amount, is the likely reason for its success (18, 34, 36). It was not economically feasible to assess a $5.00 bill, which has outperformed the $2.00 bill in other studies (33, 37). The $2.00 check, which was least effective, may have been viewed as more of a nuisance to deposit rather than a gesture of appreciation. Our finding that responders generally cashed their checks and nonresponders did not is consistent with results from other studies (16, 44, 45). The smaller percentages of responders and nonresponders who cashed the $2.00 checks versus the $5.00 checks may reflect perceptions that the small amount was not worth the effort to cash. Our higher costs per response, compared with other reports for comparable incentives sent with a first mailing (18, 33, 37, 38), likely reflect the difficulty in increasing response among subjects who had not responded to two earlier mailings and, for some, a telephone prompt.

The limitations of our study included incomplete evaluation of all permutations of amount or type of incentive, as well as delivery method, and the potential lack of applicability of the findings beyond a predominantly female, medical worker cohort. Our data cannot address the effect of the prenotification letter, because we used it with questionnaires sent by first-class mail but not with those sent by Federal Express. Despite allocating 300 persons per arm, the generally low returns among persons not responding to two previous mailings limited the power to estimate differences among certain age, gender, race, geographic, and other categories evaluated. Useful patterns may be suggested, but our results should be interpreted cautiously.

As suggested (14, 15, 20, 26), we conducted a randomized trial to test approaches for improving participation and then applied the findings in a cost-effective manner. The result was a notable improvement in the final participation rates. As information accumulates about individual and population differences in response to small incentive amounts/types and delivery methods, it may become possible to tailor cost-effective strategies for improving response rates for specific subpopulations. As response to mail solicitations continues to decline (46, 47), pilot trials to identify low-cost procedures to enhance participation may become increasingly important.


    NOTES
 
Correspondence to Michele M. Doody, Radiation Epidemiology Branch, National Cancer Institute, Executive Plaza South, Room 7088, Bethesda, MD 20892-7362 (e-mail: doodym{at}exchange.nih.gov). Back


    REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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