Since 1996 an on-going collaboration with participating member countries to develop comparative data on students with disabilities, learning difficulties and disadvantages (DDD) has been taking place. The task is complicated due to the different conceptual frameworks and definitions relating to DDD students across countries.
A significant aspect of the collaboration is a continual process of refinement of data quality and quantity, checking and consultation. Countries are using the process to improve their national data collections, to fill gaps, and to ensure greater consistency and clarity in definitions and coverage.
Data collection brings together national data sets of students in schools who are receiving additional resources to help them access the curriculum. Data are collected from countries in as disaggregated a form as possible and then grouped into broad categories based on explicit definitions and previous agreement among the countries. Details of the methodology and classifications are provided in previously published OECD works (2000 , 2004). The full country data set is provided so that other researchers can also access the data.