Purpose - This commentary discusses the paper by Reardon et al. (2024; this issue) entitled, "Overcoming implementation challenges through using a train-the-trainer approach to teach numeracy in a special school setting." This commentary outlines the necessary contribution this paper adds to the substantive area of research it is couched within while also identifying potential areas of future research to expand the understanding of this phenomenon and its impact upon practice. Design/methodology/approach - What is fidelity of implementation, how do we measure it and how does it impact our interpretations of experimental findings? This commentary focuses specifically on the nebulous construct of fidelity in experimental studies and how this impacts experimental findings' internal and external validity. Findings - Although fidelity is frequently referenced as an important aspect to consider, the measurement of the construct has been critiqued in experimental studies. To understand if an intervention was "implemented as intended," the core dimensions of the intervention must be considered in the measurement process, as well as potential confounding variables. Originality/value - With an increased need for experimental work to inform what works, for whom and under what conditions, there becomes a need to better investigate the implementation of the intervention in these contexts - thus, fidelity must be reconceptualized. This commentary provides an overview of this dilemma with potential ideas to investigate moving forward.
