Case studies allow us to gather data that is both in depth and in detail. It is either of an individual or a small group of people connected in some way. Different research methiods are used within case studies like interviews, questionaires and observations. To gather further data tests and experiments can also be used. The case history is the central research tool which provides qualitative data. An example of a case study is "Genie".
It has been argued that Case Studies are not scientific because they mainly collect qualititive data even though it is in detail and richness, it depends on how the data is interpretted.
Questionaires and interviews should be well constructed and where possible, show validity and reliability. To be valid they must measure what they claim to measure.
To be reliable they must show the same results when repeated. The more valid and reliable the data is, the data and results are genaralisable to some extent (even though not being genaralisable is often a weakness of case studies)