Overall, participation among recruited mothers was good in the HOEPS. Fathers invited to participate in the HOEPS also had high participation rates, unlike a previous study that found women were 10% more likely to complete an initial survey than men
. Fathers were lost to participation, however, when the mother could not be located, refused participation in HOEPS, or did not give permission to contact the father. Information provided from fathers directly may be more accurate than those reported by proxy, however
[20–24]. Our results show that obtaining information from fathers, in addition to mothers, is feasible though participation rates may be lower when mothers are ‘gatekeepers’ to study participation. In studies of reproductive outcomes, mothers are often contacted first: mothers are the sources of information (or permission to medical records) about pregnancy and delivery characteristics, fathers may not be specified on birth records, and biological fathers may not be aware of the birth of the child. Since mothers who resided with the index child’s biological father six months before and during the index pregnancy were more likely to give permission for researchers to contact the father, this could be an important source of selection bias in evaluating paternal exposures in studies of birth outcomes where mothers act as gatekeepers.
Fewer mothers completing the MQ gave permission to contact fathers compared to mothers completing the CATI; this was not explained by underlying differences in the percentages of mothers who reported living with their child’s biological father during the entirety of B6-T3. Mothers who received the MQ had the opportunity to ask fathers if they wanted to be contacted before giving us permission; this may explain why mothers assigned to the MQ tended to give permission less often to contact the father, but the fathers were more likely to participate when contacted. During the CATI, mothers also had an opportunity to clarify what was expected of the father (including what information would be solicited and how long the interview would take). Sending both the maternal and paternal MQs to the mother at the same time might increase paternal participation; mothers could then choose whether or not to pass the paternal MQ to the father immediately if they share a residence.
Because the HOEPS was a follow-up among previous study participants, our population is probably more motivated and the overall participation rates we observed are likely higher than would be expected in a survey within the general population. This structure, however, created an excellent opportunity for comparing participation between instruments: we had good contact information (both telephone and mailing), allowing our recruitment protocols to be similar between both instruments; all families were drawn from the same population of NBDPS participants and randomly assigned to an instrument, removing most potential confounding/bias in a comparison of the instruments; and we had demographic information on eligible non-participants, allowing us to evaluate differences in participation rates between specific subgroups. Our design may have been somewhat biased towards high participation rates among those assigned to the CATI, because participation in the NBDPS requires completing a telephone interview. All those eligible for the HOEPS therefore had telephone access and a demonstrated willingness to provide information over the telephone.
We found that participation rates were higher for both mothers and fathers assigned to the MQ versus the CATI. Participants assigned to the MQ also required fewer reminder call cycles or letters before completing the study versus participants assigned to the CATI. As we expected, the largest impact of survey method on participation rates occurred among control subjects. Case subjects are generally more highly motivated than control subjects, thus we would expect the attractiveness of a survey method to have less weight in motivating participation among cases than controls. In all demographic strata we examined, participation rates were higher for mothers assigned to the MQ versus the CATI. The distribution of participating subjects across demographic groups, however, was similar to the distribution of eligible subjects. This suggests that mothers recruited to the HOEPS are likely representative of NBDPS participants, and that the increased response to the MQ was not attributable to a specific demographic subgroup.
Future studies that evaluate the use of a MQ should consider whether the increased cost of postage and printing for a questionnaire will be offset by savings in recruitment costs and increased participation rates. We found that the MQ saved personnel time (primarily due to fewer reminders) compared to the CATI, while postage and printing costs of the MQ were only slightly increased compared to those for the CATI. Overall, the mailed questionnaire was more cost-effective for this study, though following up on missing or confusing responses to the MQ required considerable time and effort whereas interviewers could immediately probe vague answers during the CATI. Complex surveys soliciting detailed answers may therefore be better suited to a CATI than a MQ.
There are several important limitations to this study. The increased participation rates we observed among those invited to the MQ versus the CATI may not be generalizeable to all groups; our population consisted primarily of the parents of young children who might have difficulty finding quiet time to complete a telephone interview or predicting when they will have free time. A paper questionnaire might be easier for these busy parents to fill out a little at a time when opportunities arise. In this study, we were also unable to compare the impact of survey type on the validity of respondents’ answers. Research from the 1970s and 1980s found that responses to MQs were generally equivalent to
 or more valid than responses to CATIs
. A 1979 study of item omission between mailed and telephone surveys found that participants were more likely to answer sensitive questions on a mailed survey
. These studies, however, were undertaken in decades past and it may not be accurate to apply these results to current survey research; few similar studies have been conducted recently. We also did not evaluate the impact of offering more than one mode of survey instrument, which has been shown to improve participation rates
[26, 27]. As internet coverage increases, web-based surveys may also provide a practical and economical alternative.
Despite these limitations, this study provides a valuable comparison of survey instruments. Since subjects were randomized to the MQ or CATI, neither self-selection effects nor confounding is likely to affect the comparison of the two instruments. This is supported by the similar distribution of maternal demographic characteristics between mothers assigned to each instrument, as well as the similarities we observed between all those eligible for HOEPS and those who completed participation. We obtained a higher response rate with less recruitment effort using the MQ.