Final answer:
A level 3b study on the USC Evidence Pyramid is a case-control study, an observational study that compares individuals with a certain trait to those without to identify contributing factors.
Step-by-step explanation:
A level 3b study on the USC Evidence Pyramid refers to a certain type of observational study, specifically, a case-control study. This kind of study involves comparing a group of subjects who have a particular trait or condition (the cases) to a group of subjects who do not have the trait (the controls), with the aim of identifying factors that might contribute to the presence of the condition or trait. An example of a case-control study is the research conducted by Doll and Hill regarding smoking and lung cancer, whereby they retrospectively compared the smoking habits of individuals with and without lung cancer to find an association between smoking and cancer.