First, it is important to have some information on the data material that you use in your thesis. For example information on the study design, when and how the data was collected, the study population, data sources and so on. It is also important to have information on attrition/non-response/loss to follow-up. This information is appropriate to write about in the section on data material.

This thesis example is a cross-sectional study, using data from a survey conducted in Swedish schools in 2010-2011. This survey contained information on self-rated health, the school environment, as well as sociodemographic indicators. The study population were students in the 7th and 10th grade, consisting of a total study sample of 12,540 individuals.