This paper reports on a study into the untutored ability of Grade 10 students to engage in argumentation about the interpretation of experimental data, and the effect of unsupported small group discussions on this ability. In this study, argumentation intends to promote critical thinking. The sample includes 266 students from five South African schools across the resource spectrum, forming 70 friendship groups. Students are required to provide written interpretations of experimental data, and justify these interpretations based on the evidence and concepts of measurement. Individual responses form the basis of small group discussions after which students again provide written justified interpretations of the readings. The data show an initial low level of argumentation, with significant variation according to school resource levels. Considerable improvement in the level of argumentation occurs merely through small group discussions unsupported by the teacher. The findings suggest several factors influencing argumentation ability such as experience with practical work, perceptions of the purpose of small group discussions, the language ability to articulate ideas and cultural influences. Methodological issues arising from the study and implications for teaching and assessment are discussed. (Contains 4 tables.)