Effects of Mode and Transitioning to a Mixed-Mode (Web/Phone) Design on Categorical Survey Estimates: Do Question Characteristics Matter?

An increasing number of telephone surveys are introducing a web option. Limited experimental research has explored the implications of such design changes and how they vary by question characteristics. Using an experimental design and propensity score matching techniques, this study examines the effects of mode (web vs. telephone) and transitioning to a mixed-mode design (from telephone-only to the choice of web or telephone) on survey estimates and how effects differ by question characteristics. We draw upon an experiment embedded in the National Study of Caregiving (NSOC), in which half of NSOC-eligible caregivers were randomized to a telephone-only design and the other half to a sequential mixed-mode design offering web and telephone options. For each categorical survey item, we test whether responses differ significantly by mode (after adjusting for selection effects using propensity score matching) and by design (experimentally assigned). We find that for the matched sample, significant differences by mode are evident for 10% of categorical survey items. These differences are larger for the approximately 75% of subjective (vs. 25% objective) questions and 45% non-binary (vs. 55% binary) questions, but are nevertheless negligible in size (<0.07 phi coefficient or Cramer's V). Because most effects are small and only about half of those randomized to the mixed-mode design opted for web, these mode effects rarely result in differences in estimates (3%) between the telephone-only and mixed-mode designs and differences in estimates are on average negligible in size (<0.03). We demonstrate that even if web take-up rates reach 90%, significant differences in estimates between telephone-only and mixed-mode designs remain rare (5%) and on average negligible in size (0.08) for the mix of categorical questions in NSOC. We discuss implications for designing future mixed-mode surveys.