In the past decade, the numeracy component in adult basic education has gained scholarly attention. The issue has been addressed by large-scale assessments of adults' skills and intergovernmental policy agendas, but also by qualitative research into numeracy from the perspective of social practice theory. However, some aspects of numeracy are still under-researched. This article argues that adults with learning difficulties (also referred to by some as intellectual disabilities) and their numeracyrelated abilities are still hidden in the blind spot of large-scale testing regimes. To address this underrepresentation, the authors present an overview of the key paradigms of a global testing culture, outline their two areas of interest, disability and numeracy, and identify the blind spots of large-scale international surveys. They consider ways of extending the range of methodological approaches to investigating (literacy and) numeracy, and opt for a participatory research approach, for which they identify five guiding principles. They showcase and reflect on these guiding principles by presenting selected data from a small-scale qualitative study on numeracy practices of adults with learning difficulties. Their aim is to demonstrate the benefits and limitations of using participatory research approaches in the context of investigating numeracy among all members of a country's adult population.