Case series designs yield the lowest form of observational evidence. Researchers choose a series of cases in the population that share some sort of similar characteristic and then they analyze pertinent predictor, demographic, and clinical factors associated with the outcome in the group of cases.  Case series designs are at times also called pilot studies.  

Case series designs are often employed in basic science and pre-clinical research.  They are useful for generating hypotheses, effect sizes for future power analyses, and studying extremely rare outcomes.  

Case series designs are an excellent choice for novice researchers looking to get their feet "wet" in empirical pursuits. They also prove their worth when evidence-based measures of effect do not exist in the literature.  Running a small pilot study or case series can yield important measures of effect.