This paper conducts a theoretical examination of an evidence-based approach (EBA) to research on language policy and language-in-education policy (LPLEP), as well as a methodological examination of the feasibility of evidence-based LPLEP research based on actual studies. Theoretical scrutiny, based on EBA's core concepts--evidence hierarchy, internal validity, and external validity--presents a guideline that suggests what kinds of LPLEP evidence are better than others. Next, to examine how to make this type of research feasible, this paper discusses methodological issues based on two actual studies on the effectiveness of a primary school English language teaching programme implemented in Japan since the 2000s, selected because they had strong internal validity (by controlling for covariates) and external validity (by secondary analysis of a randomly extracted sample). Based on the theoretical and methodological examination, this paper claims that the EBA's core concepts are fully applicable to applied linguistics and LPLEP research, providing meaningful information to researchers and policy-makers.