@article{Schonlau2009,
author = {Schonlau, M. and Soest, A. and Kapteyn, A. and Couper, M.P.},
title= {Selection bias in web surveys and the use of propensity scores},
year = {2009},
doi = {10.1177/0049124108327128},
volume = {27},
number = {3},
pages = {291‒318},
}