Combined mobile-phone and social-media sampling for web survey on social effects of COVID-19 in Spain

To minimize coverage and participation biases, this web survey adopted two complementary sampling strategies: (a) SMS invitations to random-number mobile phones, and (b) Internet advertisements targeting specific sociodemographic groups. This combination of aleatory selection from a rather comprehensive sampling frame, on one hand, and responsive definition of target profiles, on the other, largely succeeded in adjusting the sample to relevant population parameters, albeit with the noteworthy exception of education level.