A diary study is an in-depth reflection on learning processes or teaching experiences regularly kept by an individual and then analyzed to look for recurring patterns or significant events.
Originated from both psychological and anthropological research, diary studies involve systematic personal accounts of the feelings, thoughts, beliefs, attitudes and reactions over a period of time.
They can be adopted as a research tool to gain insight into learners' language learning experiences.
Numrich in 1996, for example, found interesting issues about early preoccupations of novice ESL teachers and the reasons why they chose some instructional methods through her diary study.
The data may also be contaminated because the diarist doesn't want their thoughts and feeling to be published or they may make up their entries to cater to the researcher's needs.