So the case study is more practical than theoretical. “The case study research design has evolved in recent years as a useful tool for investigating specific trends and situations in many scientific disciplines” (Martyn Shuttleworth 2008, paragraph 1). So case study is not a rigid method. More benefits can be achieved by using many sources or other methods as long as they are relevant and focus on the specific subject. The case study design may change slightly depending on particular or specific issues as long as it can create deeper understanding and detailed knowledge and thus create solutions. But you also have to keep in mind that the case study is never about right or wrong because it is SU based