Studies of Welfare Populations: Data Collection and Research Issues. Effects on Respondents or Effects on Interviewers?

06/01/2002

Are the consistent effects of incentives in telephone and face-to-face interviews attributable to their effect on respondents, or are they, perhaps, mediated by their effect on interviewers? Clearly this question does not arise with respect to mail surveys, where incentives also have been consistently effective, but it seems important to try to answer it with respect to interviewer-mediated surveys. It is possible, for example, that interviewers expect respondents who have received an incentive to be more cooperative, and that they behave in such a way as to fulfill their expectations.(2)  Or they may feel more confident about approaching a household that has received an incentive in the mail, and therefore be more effective in their interaction with the potential respondent.

To separate the effects of incentives on interviewers from their effects on respondents, Singer et al. (2000) randomly divided all sample numbers in an RDD survey that could be linked to addresses into three groups. One third of the group was sent an advance letter and $5; interviewers were kept blind to this condition. Another third also received a letter plus $5, and still another third received the letter only. Interviewers were made aware of these last two conditions by information presented on their Computer-Assisted Telephone Interview (CATI) screens.

The results of this experiment are shown in Table 4-1. Large differences were observed between the letter-only and the letter-plus-incentive conditions, but there is no evidence that this is due to the effect of incentives on interviewers. Only one of the differences between the conditions in which interviewers were aware of the incentive and those in which they were not aware reaches statistical significance, and here the results are in a direction opposite of that hypothesized. Thus prepayment of a $5 incentive substantially increases cooperation with an RDD survey, and the incentive appears to exert its effect directly on the respondent rather than being mediated through interviewer expectations. This conclusion is in accordance with research by Stanley Presser and Johnny Blair, at the University of Maryland, who also found substantial increases in response rates as a result of small prepayments to respondents to which interviewers were blind (personal communication, n.d.).

TABLE 4-1:
Response and Cooperation Rates by Advance Letters and Letters Plus Prepaid Incentive,
Controlling for Interviewer Expectations
  Response Rate (a,b) Cooperation Rate (b,c)
Interviewed
%
Not Interviewed
% (n)
Interviewed
%
Not Interviewed

% (n)

May 1998
  • Letter only
62.9 37.1 (62) 68.4 31.6 (57)
  • Letter + $5, interviewers blind
75.4 24.6 (69) 86.7 13.3 (60)
  • Letter + $5, interviewers not blind
78.7 21.3 (61) 82.8 17.2 (58)
Ltr only vs. ltr + $5 X2=4.13, df=1, p<.05 X2=6.27, df=1, p<.05
Blind vs. not blind n.s. n.s.
June 1998
  • Letter only
58.2 41.8 (55) 62.8 37.2 (51)
  • Letter + $5, interviewers blind
73.8 26.2 (61) 86.5 13.5 (52)
  • Letter + $5, interviewers not blind
74.6 25.4 (59) 83.0 17.0 (53)
Ltr only vs. ltr + $5 X2=4.52, df=1, p<.05 X2=9.56, df=1, p<.01
Blind vs. not blind n.s. n.s.
July 1998
  • Letter only
61.8 38.2 (55) 72.3 27.7 (47)
  • Letter + $5, interviewers blind
81.3 18.6 (59) 87.3 12.7 (55)
  • Letter + $5, interviewers not blind
69.6 30.4 (56) 72.2 27.8 (54)
Ltr only vs. ltr + $5 X2=3.47, df=1, p=.06 n.s.
Blind vs. not blind n.s. X2=5.83, df=1, p<.10
August 1998
  • Letter only
63.8 36.2 (58) 69.8 30.2 (53)
  • Letter + $5, interviewers blind
75.0 25.0 (68) 81.0 19.0 (63)
  • Letter + $5, interviewers not blind
76.7 23.3 (60) 85.2 14.8 (54)
Ltr only vs. ltr + $5 X2=2.85, df=1, p=.09 X2=3.75, df=1, p=.05
Blind vs. not blind n.s. n.s.
SOURCE: Singer et al. (2000).

a  Includes noncontacts in denominator.
b  After refusal conversion.
c  Excludes noncontacts from denominator.

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