Basically, why is the quasi-experimental method of testing a hypothesis not as “true” as the true experimental method discussed earlier? Single-subject designs are not just “another way to answer a...

Basically, why is the quasi-experimental method of testing a hypothesis not as “true” as the true experimental method discussed earlier?

Single-subject designs are not just “another way to answer a question.” They, indeed, depend upon the very rationale for asking the question in the first place. What is this rationale?