Mental computation is an activity performed in the mind so written tests and work samples are usually limited in their use for assessment.
Observation and interviews are suggested as the most useful ways to gain information about students' mental strategies. These forms of assessment are not necessarily discrete since teachers in the act of observing may deliberately intervene while a student is working to ask questions.
The essential difference between observation and interviews is that an interview is an occasion where time is specifically set aside apart from the normal routine with the intention of obtaining data about one student.
Forms of interviews
Interviews are formal situations in which time is set aside to ask an individual student questions that potentially reveal their preferred mental computation strategies.
Observation is an unobtrusive form of assessment as it involves noticing what students are doing while engaged in usual learning tasks.