Answer to Question 1
(a) It is impossible to establish exactly who constitutes the population.
(b) There are privacy issues associated with the population.
Answer to Question 2
(a) Snowball sampling involves finding an initial group of participants in a hidden population, then asking them to refer others who might be of interest to the researcher.
(b) Key informant sampling involves getting information from people who have intimate knowledge of members of the hidden population but who are not themselves members of the group.
(c) Targeted sampling involves finding a location where members of the population of interest congregate, then approaching them at that location.
(d) Respondent-driven sampling involves finding an initial set of participants from a hidden population, then giving them primary (e.g., monetary) incentives and encouraging them to find others who, because of their acquaintance with the initial person and due to secondary incentives (e.g., social approval), agree to participate.