In a diary study, participants are asked to keep a diary of certain daily life events that are of interest to the researcher (Bolger, Davis, & Rafaeli, 2003). By letting the participants register these events on a regular basis (daily, weekly, whenever the event occurs, etc.), the researcher gets an idea of the frequency and context of the event. Such registrations by the participant can be done via, for example, a predefined paper-based diary, online or mobile applications, or by telephone calls. Diary study is a method that is a form of Ecological Momentary Assessment (EMA), which is characterized by the collection of data in the real world, a focus on individual’s current or recent states, and multiple assessments .
Sometimes it can be very difficult for people to recall how specific events occurred, who were involved, how exactly they were feeling at that moment, or how often something occurred. A diary study enables the participant to record the event before he/she forgets about it. In addition, when it is difficult for participants to open up about a certain event or behave differently when being observed because of shame or social desirability, a (anonymous) diary study can be a good alternative. Drawbacks of diary studies are the needed participant compliance with keeping the diary and subjectivity. However, prompts can be used to remind the participants. Furthermore, diary studies can in some cases require a substantive amount of instructions for the participants.
Diary studies can render a broad range of information: either more factual data (what, where, how often, for how long) or subjective experiences. For example, people can be asked to daily record in a diary how they feel after a knee surgery. This would render insights into the recovery and possible motivational issues during this period. Researchers may use such information to develop supporting interventions for post-operative care. In other cases, diaries can give insight into the frequency or gravity of a problem or occurrence of event, for example, the intensity and supportiveness of social contact during the daily life of cancer treatment.
 Bolger, N., Davis, A., & Rafaeli, E. (2003). Diary methods: capturing life as it is lived. Annual Review of Psychology, 54, 579-616.
 Trull, T.J., & Ebner-Priemer, U.W. (2009). Using experience sampling methods/ecological momentary assessment (ESM/EMA) in clinical assessment and clinical research: Introduction to the special section. Psychological Assessment, 21(4), 457–462.