Community-Based Participatory Research (CBPR) is often understood primarily as an ethical approach to involving vulnerable groups. This article argues that CBPR is, moreover, a central prerequisite for the methodological quality and epistemic validity of scientific research. Drawing on a detailed analysis of an empirical case from the field of sign language interpreting, it shows how missing or insufficient participation can affect various phases of the research process - from the development of the research question, through the design of the study, to data collection, analysis, and interpretation of the results.

The analysis makes clear that insufficient involvement of the affected communities can lead to fundamental methodological problems, including inadequately validated survey instruments, undifferentiated samples, and conclusions that lack a sound empirical basis. It further shows that these deficits not only limit the explanatory power of scientific findings but can also durably undermine marginalized groups’ trust in scientific institutions.

Particular attention is paid to the development and application of so-called Sign Language Oriented Easy Language Plus (SEL+), which serves as an example of how a lack of participatory coordination can lead to methodological, linguistic, and cultural tensions. The article situates this problem within the broader context of structural power relations and epistemic inequalities within the scientific system. Finally, it presents Deaf Communities-Based Participatory Research (D-CBPR) as an approach that systematically takes into account the specific linguistic, cultural, and epistemic conditions of Deaf communities. From this, the article derives the demand to establish participation as an independent criterion of scientific quality and to further develop corresponding guidelines, for example within the framework of the German Research Foundation (DFG).

Keywords: Community-Based Participatory Research (CBPR), Deaf Communities, Participatory Research, Methodological Validity, Epistemic Justice, Deaf Studies, Language and Modality, Research Ethics.