Korf, D., Benschop, A., Werse, B., Kamphausen, G., Felvinczi, K., Katarzyna, D....Van Hout, M. C. (2021). How and where to find NPS users: a comparison of methods in a cross national survey amongst three groups of current users of new psychoactive substances in Europe. International Journal of Mental Health and Addiction, 19, 873?890. https://doi.org/10.1007/s11469-019-0052-8