Someone constantly asks me how to proceed with a study in which the researcher wants to check something, for example, satisfaction with the choice of a course. It has a population (students of the course), sends questionnaires to these students, through a platform like Google Forms, and obtains a "sample". From this point I am asked with the following doubts: what is the validity of this sample? To what extent are my estimators compromised? How much bias did I introduce? Can I formulate this mathematically? (there are many doubts, but as they are linked I think it is easier to reflect)
Thanks in advance!