Purpose: This paper aims to describe the statistical arguments produced by third-grade students (8-9 years old) and to identify the teaching support for collective argumentation in a lesson based on data comparison. A Lesson Study Group researched and planned the lesson around a problem from the official mathematics textbook. Design/methodology/approach: From an interpretative perspective, we have analysed the arguments produced by students in a situation where they should give reasons to support or refute some claims based on data analysis. We looked at how some teaching support strategies promoted collective argumentation. Findings: The strategies outlined in the lesson plan enabled the teacher to foster collective argumentation, encouraging students to provide reasoning based on data analysis. The lesson plan served as a means of improving the context presented in the textbook, guiding the development of well-structured teaching, and promoting high-quality teaching practices. Research limitations/implications: One of the limitations, and future lines of research by the LSG is the deepening of teachers' understanding of the support required for CA in their classrooms so that they can distinguish the components of an argument and their role in and interpretation of the effectiveness of the arguments. In addition, the lesson plan did not consider in depth aspects such as the argumentative processes of the students, nor did it implement specific actions to promote argumentation. Addressing these limitations would be interesting and is necessary, considering that teachers still do not understand the key role of argumentation in learning and that the mastery of CA strategies is still a challenge for initial teacher training and for professional development programs for teachers. Practical implications: For research purposes, evidence is presented of the types of teacher support in collective argumentation in a comparative task of two dot plots. For teaching purposes, these types of support can be pointed out within Lesson Study Groups and included in lesson plans, allowing discussions and base-data argumentation. Social implications: The research has social implications in civic development, educational inclusion, and adaptation to technological and pandemic changes, with a focus on inferential statistical reasoning and the crucial role of the teacher in facilitating collective argumentation in the online school classroom. Originality/value: The study enriches knowledge about the potential of Lesson Study and the possibilities of planned online lessons to develop professional learning on collective argumentation with data, as exemplary teaching practices that should be widely shared.