“…It is very challenging to accurately estimate the prevalence of specific NPS classes because most of the surveys conducted focus either on NPS as a whole or on specific NPS, although some progress has been made in recent years in improving survey instruments and tools (UNODC, 2017a). Other technical issues include: self-reporting surveys, such as the Global Drug Survey (Winstock et al, 2017) may not be representative of all users; there may be differences between respondents who @ 13 November 2017 4 have internet access and chose to participate compared to those who lack such access or desire to take part (Ashrafioun et al, 2016). General household surveys which will not capture people living alone, in institutional settings (hospitals, penal establishments, student accommodation, sheltered housing, etc.)…”