The paper reports ongoing research investigating pre-service physics teachers’ reasoning when they analyse experimental data in the undergraduate physics lab courses. The focus is on how pre-service physics teachers understand mathematical expressions/equations when they analyse experimental measurements in the undergraduate physics laboratory. In particular, we are interested in how they understand slopes of best fit lines and rates of change at some points of best fit lines. The main sources of data are open-ended written tasks, semi-structured interviews and lab reports (written and submitted after each experiment) in order to understand their reasoning behind the students’ adopted approaches. In total, sixteen pre-service physics teachers participated. It was found out that the participants have had particular difficulties when they reason with mathematical expressions as they demonstrate a restricted understanding of them as formulas only. The main argument developed by this research study is that physics instructors need to give much importance to lab reports as a learning tool. In fact, lab report write-up is regarded as an integral part of the experimental process. And then, in research terms, lab reports are seen as a valuable source for pre-service physics teachers’ difficulties and full reasoning.