Non-random sample selection consists in the fact that the initially selected small group of respondents is examined and each member of the group specifies (recommends) other units of the general population to be further examined. In this way the sample size starts to grow rapidly. The main advantage of this technique of selecting a sample is that it allows it to reach individuals in the absence of a census or when it would be very difficult for them to reach the researcher (e.g., research into criminal groups, illegal associations, etc.).

• Non random sampling
• general population