Just as no model includes bad pictures in their portfolio, no company submits bad case studies with their references. Look to politics to see another common use of case studies. How often do we hear about “the mom whose life was saved by Obamacare” or “the family that had their health insurance increase by 3x by Obamacare”?
A case study is the very definition of small sample size. If someone does 100 projects and 97 are poor to fair results but 3 were superb, guess which 3 you are going to see as case studies? Guess which 3 will be used as references? Now, would you like to guess the likelihood of your project being an absolute success?
Case studies do have a role in the sales process but only with a hefty dose of salt.