Adaptive behaviour impacts the classroom's environment and the student's comfort. Therefore, a deep understanding of students' adaptive behaviour is required. This study aims to develop a valid and reliable method to realize how children in their late middle childhood (9–11) practise adaptive behaviours as a response to the classroom's Indoor Environmental Quality (IEQ). A self-reported questionnaire accompanied with an observation form is designed based on children's ‘here and now’ sensations, their cognitive and linguistic competence. Validity and reliability of the questionnaire were tested by running pilot and field studies in eight primary schools from July 2017 to May 2018. Through transverse sampling, 805 children were observed, and 1390 questionnaires were collected in 31 classrooms. Questions and responses of the designed questionnaire were validated by monitoring answer-process, non-participant observations, cross-checking questions and statistical tests. Validating process improved the wording of the questions and response categories and resulted in a questionnaire with a high and valid response rate. The reliability of the questionnaire was tested by measuring the variability and standard deviations of responses under similar conditions. To conclude, the study introduces a questionnaire and an observation form that should be used together to provide a valid and reliable method for studying adaptive behaviour of primary school children
