There was another interesting thing that happened in our mixed mode that varied the sequences of modes. As a reminder, the survey was a screening survey. It identified households with an eligible person. An interviewer later returned to eligible households to complete a 'main' interview. We found that eligible households in locked buildings that had received the FtF-Mail treatment on the screening interview responded at higher rates to the main interview. Better even than Mail-FtF. The results were significant, even when accounting for the clustering in the sample. It may be that the FtF-Mail sequence displays our earnestness to the respondent more clearly. It would be nice to replicate these results.
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