Mixed-mode research is still a hot topic among survey methodologists. At least at about every meeting I attend (some selection bias is likely here). Although we know a lot from experiments in the last decade, there is also a lot we don’t know. For example, what designs reduce total survey error most? What is the optimal mix of survey modes when data quality and survey costs are both important? And, how can we compare mixed-mode studies across time, or countries, when the proportions of mode assignments changes over time or vary between countries?