A diary study is a type of qualitative user research, which typically involves a user recording their activities, thoughts, or behaviours over time at regular intervals (for example, when you wake up, or just before bed, each day for a week, after a phone call, or each time you brush your teeth, and so on).
This type of qualitative data helps teams better understand user behaviour, especially in cases where it isn’t practical to observe users directly. A diary study enables organizations to see how the usage of a product or service varies depending on different contexts and over time.
A public transit user recording when they took public transit, along with their experiences, thoughts, conversations, and differences about each trip over a period of two weeks would be an example of a diary study for a public transit agency.