Hi,

this method of data collection is usually used when there is difficulty to identify the sample through official sources. Researcher depends on getting access to few participants relevant to the study purpose, and those participants {for different reasons they know similar cases} guide the researcher to reach them. Frequently it is encountered with stigma issue, so families hide their affected members. 

I aslo found the attached article, entitled "Adapting chain referral methods to sample new migrants: Possibilities and limitations"  which answer your question.

wish you all the success

Thank you for your useful explanation! Now I understand why the researches in medicine use it a lot. We are using it for assess a specific consumers profile.