a question on sampling procedure



I am going to conduct a research on teachers and students using mixed-methods involving questionnaire, observation and interview. Is there a way to choose participants from the observation? For example, if I observe a class and some students participate more than the others and I would like to interview them. What will be the best academic way to do so? What this process called? I have been searching, but could not find an answer.



It's selective or purposive sampling and it's generally considered bad, in the context you're seeking to generalise, since you're 'cherry picking' the participants that will give you the most feedback but there's an inherent bias.

Obviously this depends on what your fundamental research question is, if it's completely and reasonably separate from any attempt to generalise; or any attempt to ascertain the effectiveness of the teaching method/style/pedagogy, then it can be argued as valid. You may want to look at phenomenological analysis or critical realism.