I'm thinking about a challenge that I have recently faced, which may be common for the majority of researchers. What criteria do you think are important for selecting a case study from among multiple options?
I have never have any experience on index in selecting case study as most of our case studies are selected based on research project goals and on-going collaboration with current selected case study's authorities. Unless if you are starting your research from zero. Then u may need a real complex and proper selection index. But, personally I would say 'Research project's predetermined aim/selection/purpose/goals' may be the highest criteria. Other criteria may include, 'regional variety', 'method application', etc.
I think it is just a matter of money and time! The more case studies you choose the more robust the results will be.
If you have to just select a specific case study you should consider the importance of conducting research on that case study! Sometimes emergency actions need to be done in the real world and the results of your research can facilitate decision-making and investing money.
Besides, the less investigated case studies can also be chosen for discovering and being hopeful to find out unique results. On the other hand, highly discovered case studies can also be a proper selection since lots of required data is available and it can not be denied that data is everything and can accelerate your investigations.