random?sampling?and?assignments

Description
A professor once indicated to our class that random sampling is not important. He did not say much more about
this point and many people in my class thought he was crazy. We had been repeatedly told in undergraduate
classes that random sampling and random assignment were key points of research.
Why would he say such a thing and what are some arguments in favor of his statement
Additionally, what are the most important components of conducting good scientific research That is, what do
you see as the most critical aspects of having confidence in the research that you are reading