Within evidence-based education, results from randomised controlled trials (RCTs), and meta-analyses of them, are taken as reliable evidence for effectiveness -- they speak to "what works". Extending RCT results requires establishing that study samples and settings are representative of the intended target. Although widely recognised as important for drawing causal inferences from RCTs, claims regarding representativeness tend to be poorly evidenced. Strategies for demonstrating it typically involve comparing observable characteristics (e.g., race, gender, location) of study samples to those in the population of interest to decision makers. This paper argues that these strategies provide insufficient evidence for establishing representativeness. Characteristics typically used for comparison are unlikely to be causally relevant to all educational interventions. Treating them as evidence that supports extending RCT results without providing evidence demonstrating their relevance undermines the inference. Determining what factors are causally relevant requires studying the causal mechanisms underlying the interventions in question.