Keywords: Incentives, small business surveys, response rates, Survey of Small Business Finances, unit response
*Board of Governors of the Federal Reserve System, 20th and Constitution Avenue N.W., Washington, D.C. 20551. The views expressed in this paper are those of the authors. They do not necessarily reflect the opinions of the Federal Reserve Board or its staff. All remaining errors are the responsibility of the authors.
Considerable research has been conducted on the use of incentives in consumer and household surveys (see, e.g., Singer and Kulka 2001). Generally, incentives, particularly prepaid monetary incentives, are likely to increase response rates. This finding differs little by mode of administration - mail, telephone, or in-person. However, much less is known about the use of incentives in business surveys.
In 2003, the Survey of Small Business Finances (SSBF), conducted by the Federal Reserve Board, implemented the use of incentives to increase response rates. This study examines the effects of some of the characteristics of the implementation - such as level of effort, time in queue, and consecutively-increasing incentive amounts - on unit response. Specifically, during the screening phase of the 2003 survey, all eligible firms were offered $50 for completing the main interview. As the interviewing progressed, interim response rates were lower than anticipated and an effort was made to increase response rates by increasing the incentives offered to those respondents that had been contacted but had not yet completed the interview.
We model the relationship between unit response and the incentive-offer function, controlling for a variety of firm and calendar differences. Our estimates suggest that as the number of days increase between the initial screener and main interview, the probability of completion decreases. Similarly, as the number of days increases between each consecutive incentive offer the probability of completion decreases. Additional effort, as measured by additional calls, increases the probability of completion. Finally, each consecutive offer after the initial offer decreases the probability of completion.
In the next section, some background information about the Survey of Small Business Finances is provided, followed by a short literature review of the effect of incentives on response rates. Section 4 includes the 2003 observational experiment and data construction followed by a description of the estimation model and results. The paper concludes with a summary and recommendations.
One of the planning goals of the 2003 SSBF was to improve response rates. The major innovation was to offer all respondents a monetary incentive upon completion of the main interview.3 The initial value of the incentive was $50. As the survey progressed, it became apparent that both precision goals and sample size goals were unlikely to be met with the realized unit response. In order to meet the original survey goals, it was decided to increase the incentive, first to $100, then to $200, and towards the end of the field period to as much as $500.4 At the conclusion of the survey, incentives and other efforts to increase response rates resulted in an overall response rate of just over 32 percent, which was about the same level as achieved in 1998. It is important to note that this was an observational experiment. The incentive plan was not designed to evaluate the effectiveness of incentives or different incentive levels. The 2003 SSBF incentive program did not contain a zero incentive group and the incentive groups were not mutually exclusive.
Incentives are increasingly used in surveys today. Much of the literature on the effects of incentives has been gathered in regard to mail surveys of consumers or households (Singer and Kulka, 2002). More recently, the literature has examined telephone or in-person surveys of consumers and households. Much less is known about the effect of incentives in business or establishment surveys.5 In general, incentives raise overall response rates. Kalka emphasized they work as effectively for hard-to-reach respondents (examples include homeless, young males, professional elites, and minorities). The business owner population could be considered a hard-to-reach respondent similar to professional elites (Kalka, 1995). Church (1999), analyzing 38 mail surveys, concludes that prepaid incentives yield higher response rates than promised incentives and promised incentives- that are contingent on completing the survey - do not significantly increase response rates. Also, prepaid monetary incentives yield higher response rates than non-monetary incentives and response rates increase with increasing amounts of money. At some point, ever increasing incentives fail to be effective in raising response rates (James and Bolsein, 1992).
In a study focusing on 39 telephone and face-to-face interviews, Singer, et al (1999) finds similar results. Specifically, incentives improve response rates in telephone and face-to-face interviews and the effects are similar by mode of survey administration. Response rates tend to increase with the size of the incentive. Prepaid incentives tend to generate higher response rates than promised incentives and money is more effective than gifts. Incentives have greater effects on response rates in high-burden surveys than in low-burden surveys although both benefit from incentives. And incentives have a greater effect in surveys where the response rate without an incentive is low.
Much less is known about the use of monetary incentives for businesses or establishments, although what is known is generally consistent with results for consumers and households. Jobber and O'Reilly (1998) found monetary incentives increased response rates in industrial mail surveys. Small up-front payments were more effective than a large promise for completion and non-monetary incentives had a beneficial but smaller effect than monetary incentives. White and Luo (mail surveys to convenience stores) and Gunn and Rhoades (telephone interviews with physicians) also found that prepaid monetary incentives increased response rates. Bitler and Wolken (2001) found that among respondents completing the main interview, those receiving a $20 prepaid incentive in the 1998 SSBF required fewer calls to complete the interview than those not receiving the incentive.
In an attempt to increase cooperation and boost flagging response rates, the monetary incentive offered to respondents increased over time. For the most part, the increase in incentives adhered to the following pattern: (1) Upon completion of the screener, most respondents were offered $50 (or the D&B package); (2) after several attempts to complete all cases in the batch were made, batches 1, 2, and 3 were subsampled and cases that were "subsampled in" were offered $100; (3) batch 4 cases that were sent a refusal conversion letter were offered $200; (4) all cases from all batches still being fielded as of January 6, 2005 were offered $200; and (5) all batch 1 - 3 cases still being fielded as of January 19, 2005 were offered $500.
As a baseline, the "rules" outlined above were applied to all cases, using information from the sample control file to determine each case's subsampling status. However, due to the different times at which screening interviews were completed and special interventions by interviewers and supervisors, not all cases were offered all of the incentives and the timing of when the differing incentives were offered varied. To appropriately determine whether or not a given firm was offered a particular incentive and the date on which that incentive was offered, data were taken from several different sources. NORC provided us with a list of the highest offer made to each firm and a list of cases that received refusal conversion letters. The call history file provides a complete list of all calls made to each case as well as the disposition for that call. The call text file provides the call notes written by interviewers to document relevant information obtained during the call. It should be noted that not all calls in the call history file produced a companion call note and not all notes in the text file had a companion call. For example, calls to a busy line would produce a call in the call history file with a disposition of "busy," but would generally not generate a call note. On the other hand, supervisory review of a case might produce a note in the call text file instructing the next interviewer to offer the respondent an increased incentive which would not produce an entry in the call history file.
Taken together, the information from these sources allowed us to construct a number of variables for each firm with which a main interview was attempted. First, a series of dummy variables was constructed to indicate whether or not the firm had been offered a particular monetary incentive. Second, a companion dummy was created for all firms that had been offered a particular incentive that indicated whether or not the firm had completed the interview while that offer was in play. Finally, the date that the offer was made to the firm was determined. From this information and the date that the screening interview was completed, we were able to construct the number of calls that were placed and the number of days while a given offer was in play. We could also determine the number of calls made and days that had elapsed prior to the (current) offer being made.
In addition to the information on the size of the incentive offered and the timing of that incentive, data from the D&B file and screening interview were also merged onto the final dataset. In the screening interview, information was collected on the firm's size, primary industry and organizational structure. Additionally, we have information on how difficult it was to complete the screener with the firm that we believe controls for the firm's underlying propensity to complete any survey. This includes the number of calls and days it took to complete the screener and an indicator of whether a proxy completed the screening interview because the owner could not be reached. From the D&B file, we have added the firm's credit score. Additional dummy variables were constructed to capture calendar differences in when the screener was completed
Table 1 provides summary statistics on the variables available for analysis. At first glance, higher incentives appear to be associated with lower response rates. Thirty-five percent of firms completed the interview when offered $50, whereas only 11 percent of firms completed the interview when offered $500. However, it is important to keep in mind that these are not independent samples of firms. All firms that were offered $500 had also been offered a smaller offer previously and failed to complete the interview for the smaller offer, indicating that these firms may be inherently less prone to complete the survey. There is further evidence of this in the amount of effort required to complete the screening interview. Firms that completed the main interview for the $50 incentive required the fewest numbers of calls and fewest days to complete the screener. Differences in calling protocols are also evident across incentives offered. For the $50 incentive, the average firm had 53 days and 15 call attempts to complete the main interview. For the $500, the average firm had just over five days and 2 calls to complete the interview.
There are little to no observable differences in the Census divisions and urban and rural locations of firms across the different incentives offered. There also seem to be few differences across firm ownership and industry. There are slight differences in the offers by the size of the firm, with the average firm size growing slightly as the incentives increase.
In practice, we do not observe the probability that a given firm will participate, but rather whether or not a given firm does participate in the survey.
As part of the survey sample design, after a period of attempting to field all cases in a batch, 40 percent of incomplete cases were subsampled out and no further effort was put toward completing these cases.7 In order to account for this sample design feature, we weighted observations according to the following rules. All cases that were completed prior to subsampling were assigned a weight of one. All cases that were subsampled out were assigned a weight of zero. All cases that were subsampled in were assigned a weight of (1/.6). All batch 4 cases were assigned a weight of one.8
Equation (3) was estimated separately with a probit model four times; one for each of the incentive amounts offered. Due to the nature in which the incentives were offered, these models will have a few notable characteristics. First, each model is estimated independently of the others; it is not possible to estimate a nested system because the incentives were not offered sequentially to all firms. For example, many firms were offered $200 without first having been offered $50 or $100. Second, the sample of firms is not identical across models. For example, firms that completed the survey when the $50 incentive was offered were not subsequently offered to the $100 incentive; such firms will be included in the $50 probit, but not the $100, $200, or $500 probits. Finally, while the control variables in each model are similar and aimed at capturing the same effects, they are slightly different across models. To see this more clearly, consider time in queue. When the first offer is made (most often $50), no time has elapsed. Since no firm was offered less than $50, all firms had zero days between the time the screening interview was complete and the first offer was made. The same thing is true of the number of calls; no call attempts were made prior to the first offer. Because there is no variation in these measures, their coefficients cannot be estimated in the $50 model. Similarly, because the incentives offered to a firm only increased over time (no firm was initially offered $500 and then subsequently offered $50), the coefficients for having been offered $100 or $200 cannot be estimated in the $50 model. Conversely, because not all respondents who received the $200 incentive were initially offered $50, the coefficient for the $50 incentive can be estimated in the $200 model.
The coefficients from these estimates are found in Table 2. Across all models, we find that the more time that elapses from the time the incentive is offered ("Days between $xx offer and..."), the less likely it becomes that the firm will complete the interview. Similarly, time elapsed since the screener completion prior to making the current offer ("Days since screener before...") also decreases the likelihood of completion. We also find that the more calls made while a particular offer is in effect ("Calls made between $xx..."), the more likely the firm is to complete the interview. Again, this is a result constant across all incentive sizes. Calls made prior to making the current offer ("Calls made prior to $xx offer") also enter in positively to the probability of completion, although not always significantly. In the models for the $200 and $500 incentives, we also note that having previously offered the respondent a lower incentive ("R offered $xx incentive") decreases the likelihood that the firm will eventually complete the survey, perhaps indicative of a loss in good will from the respondent. Alternatively, this could indicate that these firms are not sensitive to incentives, regardless of their size. Because there is no control group, i.e., no firm was not offered an incentive, we cannot identify whether this was the case.
Other variables enter into the likelihood much as predicted. Variables that indicate that the firm may be inherently more difficult to interview, such as having completed the screening interview with a proxy and the number of days and calls necessary to complete the screener, generally decrease the likelihood of completion.
Table 3 provides a measure of by how much the likelihood of completion is affected by the effort expended on the case. The results indicate that each day that passes with an offer outstanding diminishes the likelihood of completion by around one percent, with each day that elapsed prior to the current offer being made decreasing the probability by between .1 and .4 percent. They also show that each additional call made while the offer is in play increased the likelihood of completing the interview by between .1 and .7 percent, slightly less than each elapsed day reduced the likelihood in each specification. Each call made prior to the current offer increased the likelihood of completion by .1 percent. Finally, estimates predict a 5 to 7 percent decrease in the likelihood of completion when the firm was offered a lower incentive prior to the current one.
While Table 3 gives a feel for how effective differing measures of effort are on the likelihood of completion, they are only benchmarks for the impact when all variables are measured at their mean and do not provide any information on how this changes as the number of calls and days increase. Figures 1 through 4 look at the predicted probability of completion by the number of days since the incentive was offered. Figure 1 indicates that once the $50 incentive has been available and not taken by a firm for more than 100 days, there is almost no likelihood that the firm will complete the interview, with a very steep drop-off in the likelihood between 50 and 100 days. Examining the same pictures for $100 and $200 incentives, we see a similar pattern with the drop-off occurring more gradually, but earlier. This difference can likely be attributed to the fact that these are the same respondents who did not complete the interview for smaller incentive offers--there has already been a significant amount of time since completing the screener. Figure 4 does not look like the previous three figures, with little discernable pattern. Two factors likely account for this difference: the number of observations and the limited number of days over which the incentive was available. The $500 offer was only made in the very last days of fielding in a last-minute effort to increase response rates before coming out of the field. It is likely that a pattern similar to the earlier patterns would have evolved were similar numbers of days elapsed.
Figures 5 through 8 look at the probability of completion by the number of calls since the incentive was offered. These figures have less of a pattern than the previous four, but do indicate that the effectiveness of more calls to reluctant respondents falls; more than 20 calls seem to only marginally affect the likelihood of completion. It is important to keep in mind that calls are a much less precise measure of effort than days. The measure of calls we have counts a busy signal the same way as it does contact with the respondent, although we would definitely not consider them to have the same impact on the respondent's likelihood of participation.
|Variable||Incentive Offered ($50)||Incentive Offered ($100)||Incentive Offered ($200)||Incentive Offered ($500)|
|Incentives: Interview complete for $xx incentive||0.35||0.19||0.13||0.11|
|Incentives: Days between $xx offer and next offer/complete/end||53.45||48.89||12.99||5.56|
|Incentives: Calls made between $xx offer and next offer/complete/end||15.15||10.17||3.84||2.61|
|Incentives: Days since screener before R offered $xx||77.14||89.28||148.70|
|Incentives: Calls made prior to $xx offer||21.83||22.78||35.89|
|Incentives: R offered $50 incentive||1.00||0.95||1.00|
|Incentives: R offered $100 incentive||0.46||0.84|
|Incentives: R offered $200 incentive||1.00|
|Screener Difficulty Measures: Proxy completed screener||0.31||0.40||0.40||0.43|
|Screener Difficulty Measures: Calls made to complete screener||7.51||8.95||9.31||9.77|
|Screener Difficulty Measures: Days to complete screener||16.33||20.17||22.45||22.41|
|Screener Difficulty Measures: Screener completed Jun/Jul||0.32||0.43||0.17||0.35|
|Screener Difficulty Measures: Screener completed Aug/Sep||0.29||0.40||0.24||0.41|
|Screener Difficulty Measures: Screener completed Oct/Nov||0.30||0.17||0.38||0.25|
|Screener Difficulty Measures: Screener completed Dec/Jan||0.10||0.00||0.21||0.00|
|Screener Difficulty Measures: $xx incentive in effect b/t Nov 21 - Jan 1||0.44||0.78||0.15||0.00|
|Location Information: Urban||0.84||0.86||0.86||0.86|
|Location Information: New England||0.06||0.06||0.06||0.06|
|Location Information: Middle Atlantic||0.13||0.13||0.14||0.13|
|Location Information: East North Central||0.14||0.14||0.14||0.14|
|Location Information: West North Central||0.07||0.07||0.06||0.06|
|Location Information: South Atlantic||0.18||0.19||0.19||0.21|
|Location Information: East South Central||0.05||0.05||0.05||0.05|
|Location Information: West South Central||0.11||0.12||0.11||0.12|
|Location Information: Mountain||0.08||0.07||0.07||0.07|
|Location Information: Pacific||0.17||0.17||0.17||0.17|
|Variable||Incentive Offered ($50)||Incentive Offered ($100)||Incentive Offered ($200)||Incentive Offered ($500)|
|Organizational Type: Proprietorship||0.33||0.30||0.31||0.28|
|Organizational Type: Partnership||0.07||0.07||0.07||0.06|
|Organizational Type: Corporation||0.60||0.64||0.63||0.65|
|Industry: Construction and mining||0.12||0.14||0.13||0.15|
|Industry: Wholesale trade||0.07||0.07||0.07||0.07|
|Industry: Retail trade||0.20||0.20||0.21||0.21|
|Industry: Insurance & real estate||0.06||0.05||0.05||0.05|
|Industry: Business services||0.20||0.20||0.20||0.19|
|Industry: Professional services||0.19||0.17||0.17||0.17|
|Batch: Batch 1 case||0.24||0.38||0.14||0.32|
|Batch: Batch 2 case||0.24||0.33||0.18||0.30|
|Batch: Batch 3 case||0.24||0.29||0.22||0.38|
|Batch: Batch 4 case||0.29||0.00||0.46||0.00|
|Firm Demographics: D&B credit score||56.34||55.00||55.01||54.49|
|Firm Demographics: Total employees||41.03||46.95||45.96||48.74|
Notes: Observations are weighted to reflect subsampling. Firms that completed prior to subsampling were given a weight of 1; firms subsampled in were given a weight of (1/.6); and firms subsampled out were given a weight of 0.
|Incentive Offered ($50)||Incentive Offered ($100)||Incentive Offered ($200)||Incentive Offered ($500)|
|Effort: Days between $xx offer and next offer/complete/end||-0.060||-0.060||-0.065||-0.119|
|Effort: Days between $xx offer and next offer/complete/end (standard error)||(0.002)***||(0.004)***||(0.004)***||(0.021)***|
|Effort: Calls made between $xx offer and next offer/complete/end||0.023||0.051||0.043||0.078|
|Effort: Calls made between $xx offer and next offer/complete/end (standard error)||(0.003)***||(0.010)***||(0.006)***||(0.019)***|
|Effort: Days since screener before R offered $xx||-0.013||-0.032||-0.018|
|Effort: Days since screener before R offered $xx (standard error)||(0.004)***||(0.005)***||(0.008)**|
|Effort: Calls made prior to $xx offer||0.002||0.006||0.003|
|Effort: Calls made prior to $xx offer (standard error)||(0.004)||(0.003)*||(0.003)|
|Effort: R offered $50 incentive||-0.025|
|Effort: R offered $50 incentive (standard error)||(0.148)|
|Effort: R offered $100 incentive||-0.375||-0.387|
|Effort: R offered $100 incentive (standard error)||(0.113)***||(0.106)***|
|Participation Propensity: Proxy completed screener||-0.461||-0.020||-0.336||-0.340|
|Participation Propensity: Proxy completed screener (standard error)||(0.049)***||(0.093)||(0.067)***||(0.106)***|
|Participation Propensity: Calls made to complete screener||-0.017||0.005||-0.012||-0.006|
|Participation Propensity: Calls made to complete screener (standard error)||(0.005)***||(0.009)||(0.007)||(0.010)|
|Participation Propensity: Days to complete screener||-0.042||-0.010||-0.026||-0.021|
|Participation Propensity: Days to complete screener (standard error)||(0.002)***||(0.005)*||(0.005)***||(0.008)**|
|Firm Specific Information: Urban||-0.049||-0.175||0.036||-0.128|
|Firm Specific Information: Urban (standard error)||(0.054)||(0.114)||(0.079)||(0.137)|
|Firm Specific Information: Partnership||-0.176||-0.274||-0.019||-0.014|
|Firm Specific Information: Partnership (standard error)||(0.095)*||(0.205)||(0.117)||(0.202)|
|Firm Specific Information: Corporation||-0.006||0.116||-0.092||-0.021|
|Firm Specific Information: Corporation (standard error)||(0.047)||(0.105)||(0.069)||(0.114)|
|Firm Specific Information: Manufacturing||0.149||0.093||0.144||0.342|
|Firm Specific Information: Manufacturing (standard error)||(0.087)*||(0.168)||(0.123)||(0.207)*|
|Firm Specific Information: Transportation||0.221||0.502||0.040||0.422|
|Firm Specific Information: Transportation (standard error)||(0.106)**||(0.230)**||(0.158)||(0.269)|
|Firm Specific Information: Wholesale trade||0.082||0.145||0.073||0.339|
|Firm Specific Information: Wholesale trade (standard error)||(0.098)||(0.200)||(0.140)||(0.233)|
|Incentive Offered ($50)||Incentive Offered ($100)||Incentive Offered ($200)||Incentive Offered ($500)|
|Retail trade (standard error)||(0.075)||(0.157)||(0.105)||(0.183)|
|Insurance & real estate||0.071||0.119||0.089||0.485|
|Insurance & real estate (standard error)||(0.102)||(0.233)||(0.147)||(0.248)**|
|Business services (standard error)||(0.075)||(0.153)||(0.106)||(0.182)|
|Professional services (standard error)||(0.077)*||(0.157)||(0.111)||(0.186)**|
|Screener completed Aug/Sep||-0.026||-0.091||-0.095||0.168|
|Screener completed Aug/Sep (standard error)||(0.104)||(0.172)||(0.202)||(0.199)|
|Screener completed Oct/Nov||-0.366||-0.108||0.141||0.421|
|Screener completed Oct/Nov (standard error)||(0.151)**||(0.261)||(0.282)||(0.302)|
|Screener completed Dec/Jan||-0.143||0.198||-0.766|
|Screener completed Dec/Jan (standard error)||(0.203)||(0.350)||(1.137)|
|$xx incentive in effect b/t Nov 21 - Jan 1||-0.081||0.276||1.093|
|$xx incentive in effect b/t Nov 21 - Jan 1 (standard error)||(0.082)||(0.208)||(0.133)***|
|Batch 2 case||0.217||-0.516||-1.252||-0.308|
|Batch 2 case (standard error)||(0.092)**||(0.179)***||(0.249)***||(0.327)|
|Batch 3 case||-0.012||-2.181||-2.580||-1.516|
|Batch 3 case (standard error)||(0.137)||(0.294)***||(0.445)***||(0.735)**|
|Batch 4 case||-1.628||-3.848||-1.304|
|Batch 4 case (standard error)||(0.185)***||(0.598)***||(1.318)|
|D&B credit score||0.001||-0.001||0.001||-0.001|
|D&B credit score (standard error)||(0.001)**||(0.001)||(0.001)||(0.002)|
|Total employees (standard error)||(0.000)***||(0.001)*||(0.000)||(0.001)|
|Constant (standard error)||(0.153)***||(0.483)***||(0.920)***||(1.778)*|
Notes: Observations are weighted to reflect subsampling. Firms that completed prior to subsampling were given a weight of 1; firms subsampled in were given a weight of (1/.6); and firms subsampled out were given a weight of 0. Chi-squares tests for joint significance of Census divisions were insignificant and dropped from final models presented here. Standard errors in parentheses. * significant at 10%; ** significant at 5%; *** significant at 1%.
|Incentive Offered ($50)||Incentive Offered ($100)||Incentive Offered ($200)||Incentive Offered ($500)|
|Days between $xx offer and next offer/complete/end||-0.019||-0.006||-0.009||-0.018|
|Days between $xx offer and next offer/complete/end (standard error)||(0.001)***||(0.001)***||(0.001)***||(0.003)***|
|Calls made between $xx offer and next offer/complete/end||0.007||0.005||0.006||0.012|
|Calls made between $xx offer and next offer/complete/end (standard error)||(0.001)***||(0.001)***||(0.001)***||(0.003)***|
|Days since screener before R offered $xx||-0.001||-0.004||-0.003|
|Days since screener before R offered $xx (standard error)||(0.000)***||(0.001)***||(0.001)**|
|Calls made prior to $xx offer||0.000||0.001||0.001|
|Calls made prior to $xx offer (standard error)||(0.000)||(0.000)**||(0.000)|
|R offered $50 incentive||-0.003|
|R offered $50 incentive (standard error)||(0.021)|
|R offered $100 incentive||-0.050||-0.069|
|R offered $100 incentive (standard error)||(0.015)***||(0.022)***|
Notes: Observations are weighted to reflect subsampling. Firms that completed prior to subsampling were given a weight of 1; firms subsampled in were given a weight of (1/.6); and firms subsampled out were given a weight of 0. Chi-squares tests for joint significance of Census divisions were insignificant and subsequently dropped from final models presented here. Standard errors in parentheses. * significant at 10%; ** significant at 5%; *** significant at 1%. Marginal effects calculated at sample means.
Figure 1: Predicted probability of completion for $50 by days since offer
Figure 2: Predicted probability of completion for $100 by days since offer
Figure 3: Predicted probability of completion for $200 by days since offer
Figure 4: Predicted probability of completion for $500 by days since offer
Figure 5: Predicted probability of completion for $50 by calls made since offer
Figure 6: Predicted probability of completion for $100 by calls since offer
Figure 7: Predicted probability of completion for $200 by calls since offer
Figure 8: Predicted probability of completion for $500 by calls since offer
Bitler, Marianne P., Alicia M. Robb and John D. Wolken (2001) "Financial services used by small businesses: Evidence from the 1998 Survey of Small Business Finances," Federal Reserve Bulletin, 87: 193-205.
Bitler, Marianne P. and John D. Wolken (2001), "Effects of the level of interviewer effort on the characteristics of completed responses: An experiment using the 1998 Survey of Small Business Finances," Federal Committee on Statistical Methodology, Statistical Policy Working Paper 34, Part 5, pp. 90-99.
Church, Allan H. (1999) "Estimating the effect of incentives on mail survey response rates: A meta-analysis," Public Opinion Quarterly, 57:467-491.
Dennis, William J. Jr. (2003) "Raising response rates in mail surveys of small business owners: Results of an experiment," Journal of Small Business Management, 41:278-???, July 2003.
Gunn, W. J., and I. N. Rhodes (1981) "Physician response rates to a telephone survey: Effects of monetary incentive level," Public Opinion Quarterly 45: 109-115.
James, Jeanine and Bolsein. (1992) "Large monetary incentives and their effect on mail survey response rates," Public Opinion Quaterly 56: 442-453.
Jobber, D., and D. O'Reilly (1998) "Industrial mail surveys: A methodological update," Industrial Marketing Management 27: 95-107.
Kalka, Richard (1995) "The use of incentives to survey `hard-to-reach' respondents: A brief review of empirical research and current research practices. Seminar on new directions in statistical methodology, working paper 23, pp. 256-299
Mach, Traci L., and John D. Wolken (2006) "Financial services used by small businesses: Evidence from the 1998 Survey of Small Business Finances," Federal Reserve Bulletin, 92: 167-195.
National Opinion Research Center (2001), "The 1998 Survey of Small Business Finances Methodology Report," April, http://www.federalreserve.gov/pubs/oss/oss3/ssbf98/ssbf98home.html#ssbf98results.
National Opinion Research Center (2005), "The 2003 Survey of Small Business Finances Methodology Report," April, http://www.federalreserve.gov/pubs/oss/oss3/ssbf03/ssbf03home.html#ssbf03results.
Singer, Eleanor, Nancy Gebler, Trivellore Raghunathan, John Van Hoewyk and Katherine McGonagle (1999) "The effect of incentives in interviewer-mediated surveys," Journal of Official Statistics, 15:217-230.
Singer, Eleanor and Richard A. Kulka (2002) "Paying respondents for survey participation," in Studies of Welfare Populations: Data Collection and Research Issues, eds. Michele Ver Ploeg, Robert A. Moffitt and Constance F. Citro, National Academy Press, Washington, D.C., 2002, pp. 105-127
White, Glenn D. and Luo, Amy (2005). Business survey response rates-Can they be improved? The American Statistical Association Proceedings of the Section on Survey Research Methods, pp 3666-368.