R. Campbell. Popular Support for Democracy in Unified Germany: Critical Democrats, Springer International Publishing, Cham, First published online: February 06, 2019, https://doi.org/10.1007/978-3-030-03792-5_4. (ALLBUS).(2019)
M. Tverdostup, and T. Paas. International Journal of Economic Sciences, 7 (2):
119-142(2018)http://iises.net/international-journal-of-economic-sciences/publication-detail-6942?download=7. (PIAAC).