The findings from evaluations of parenting programmes can help inform policy and practice decisions, including how best to allocate scarce resources designed to support families. However, studies often fail to consider the extent to which the findings can be generalised to other settings or populations. One, yet unexplored solution, is to compare study findings and large-scale datasets including publicly available population data. The aims of this study were to assess the feasibility of assessing sample representativeness using publicly available data and to identify the challenges involved in considering the generalisability of study findings. Sociodemographic data from two community-based evaluations of parenting programmes conducted in England and Ireland between 2015-2018 were used in the study (N =395 parent-infant dyads). The results indicated some differences between the trial samples and the wider population. However, it is difficult to reach definitive conclusions about these findings due to the limitations associated with using the comparative datasets. Our study revealed three key challenges, including: (1) how best to define and conceptualise representativeness; (2) the availability of comparative datasets; and (3) the quality of the available data. Our study suggests that there is a need for up-to-date, good quality comparative datasets to allow for the assessment of representativeness. Further work is required to identify parameters for making claims about representativeness, specifically regarding the acceptable level of difference between the target population and the study sample. This is the first study to explore the feasibility of using publicly available population data in two jurisdictions, for the purpose of making judgements about the representativeness of the findings from parent programmes. It is hoped that our results will encourage further investigation around the reporting of trial external validity to enable effective decision making at policy and practice level.