Randomized controlled trials are not always possible when investigating the effects of a treatment. This can be due to ethical reasons, for example, it would be unethical to randomly allow or disallow mothers to breastfeed their children due to the variety of health benefits to both mother and child, which are known to come about from breastfeeding. This case study illustrates one way in which observational data can be used in the absence of a randomized controlled trial to investigate the effects of breastfeeding on body mass index during childhood. It will outline the important things to consider when using propensity score matching to mimic a randomized controlled trial. The case study will first provide an overview of the research question, the reasons that a randomized controlled trial could not be used, and the reasons for the use of propensity score matching. It will then provide further information on the decisions made while exploring the data and carrying out the analysis, problems encountered during the data analysis, as well as lessons learned during the research.