**3.1 Study design**

The study conducted during the SIALON project was a cross-sectional study targeting MSM who have had sex with another man during the last 12 months. The data collection was conducted according to the Time-Location (or Time-Space) Sampling (TLS) procedure. This method was used to recruit men visiting the selected gay settings in each participating city, allowing to construe a representative sample with known properties. The cross-sectional study was conducted using a self-administered pen-and-paper questionnaire and an oral fluid collector to gather both behavioural and biological data for each participant.

The study design was planned and implemented in line with the Second Generation Surveillance System (SGSS) criteria (UNAIDS & WHO, 2002, 2005) and the United Nations General Assembly Special Session (UNGASS) Indicators (UNAIDS & WHO, 2009).
