Individual differences Studies
A great deal of psychological research takes what is called a nomothetic approach; it tries to find things that are similar for all people so that we can find laws of behaviour that can be generalised to everyone. But isn't it a little boring treating everyone as being the same when there is so much variety in the world? The individual differences approach takes an ideographic view of people, looking to investigate the things that make people different, or even unique.
Strengths of the Individual Differences approach
Useful applications to the real world - Rosenhan's study led to changes in the way that some mental illnesses are diagnosed.
Case studies such as the one by Thigpen and Cleckley provide large amounts of richly detailed information about an individual or small group. Detailed study of an individual can be very useful for them to be given access to the appropriate treatments or help.
Some of the methods used by the approach are objective and quantifiable, such as the use of psychometric tests (IQ and memory tests) by Thigpen and Cleckley allows easy comparison between people and avoids bias
Weaknesses of the Individual Differences approach
The use of case studies can mean that conclusions may be biased, as they will often rely on the researcher's opinion. Thigpen and Cleckley made many assumptions about Eve based on Freudian theories that may well not have been accurate.
As they may study very small numbers of people, there is the question about whether the results of an individual differences investigation can be usefully generalised. If not, it could be argued that spending so much time and money finding out about one individual is a waste.
Case studies often raise ethical issues to do with confidentiality, as the individual may be easily recognisable from the case. Thigpen and Cleckley's case study raises other ethical issues as well, such as protection from harm.
Other types of study, such as the covert observation used by Rosenhan, raise clear issues of consent.
Researchers will often rely on the patient's self-report for information about a condition, but self-report data can be biased, due to social desirability bias, for example.