statistics are increasingly available.” This is, in part, due to the growing number of countries that collect disability data in censuses. At least 120 of 214 countries or regions that conducted a census during the 2010 round included a set of questions on disability, a significant increase from approximately 19 countries or areas that did so during the 1970 census round. A major challenge though is how to collect the disability data that are needed to inform policy and advocacy efforts as part of the CRPD and the 2030 Agenda for Sustainable Development. As with any complex social science phenomenon, there are several options when it comes to the scale and methods for data collection efforts. One option is to have a qualitative and participatory effort involving multiple stakeholders including persons with the lived experience of a disability. It may go a long way in understanding the situation of persons with disabilities and informing policy and advocacy efforts at one place and one point in time. However, it would not provide nationally representative or internationally-comparable data that are needed to ensure that persons with disabilities worldwide are not left behind post-2015. Another option is to develop a national disability survey with a battery of questions to measure disability and identify service needs and barriers, as well as to understand the factors leading to disability related inequalities. This is suitable provided significant resources and capacity for data collection are available and have the potential to provide very rich information to inform national or even subnational policy and advocacy efforts. The World Bank and the World Health Organization have developed a Model Disability Survey to this end (Cieza et al 2018; Groc 2019; WHO 2020). It has a full stand-alone survey which takes 60 to 120 minutes to accomplish. It also has a brief version, taking an average of 10-15 minutes to complete and includes over 40 questions on environmental factors, functioning and health conditions. Few countries may, however, have the capacity or resources needed to develop and implement the stand-alone survey or include the brief version in an existing survey, let alone repeat this over time for monitoring the situation of persons with disabilities post-2015 and in the context of the CRPD.

Yet another option consists in using a few questions to measure disability in a population census as well as in general or mainstream household surveys, such as the Living Standards Measurement Study. A population census or a mainstream survey makes it possible to measure prevalence as well as inequalities across disability status for general indicators such as employment rates and educational attainment. However, unlike the participatory and disability surveys above, it will not allow for an in-depth analysis of disability-specific barriers or challenges, nor facilitate understanding of the drivers of inequalities. However, it may give very useful information when it comes to tracking the situation of persons with disabilities in different aspects of their lives. Of course, measuring disability with a few questions is challenging, especially if it has to be done in an internationally-comparable manner for global SDG and CRPD related monitoring efforts. Since 2001, there has been an international effort to collect internationally comparable disability data by the United Nations’ Washington Group on Disability Statistics (the “Washington Group'' (WG)). This group, organized under the United Nations Statistical Commission, gathers over 100 representatives of National Statistical Offices as well as other government, non-government, and disabled people’s organizations. There was a broad consensus on the need for population-based measures of disability for country use and for international comparisons and the Washington Group on Disability Statistics was formed to address this urgent need. The main purpose of the Washington Group is to promote and coordinate international cooperation in the area of disability measures. The Washington Group has developed and tested questions and made recommendations for a short set of six questions 3