The words in a language or language variety are often rank ordered in lists that are meant to reflect the relative importance of those words to language users and learners of a language. This rank ordering is done on the basis of the relative prevalence of words in a corpus. Lexical prevalence is often operationalized as measures of frequency, dispersion, or adjusted frequency. Yet, to date, there is no consensus on best practices for identifying and ranking prevalent words in a corpus, or for evaluating the degree to which a word's importance is reflected through its prevalence. We begin this paper by introducing and describing a wide range of corpus-based measures for quantifying lexical prevalence. We then carry out two case studies on the Duolingo University Textbook Corpus to evaluate the methods for their ability to identify and appropriately rank words in terms of their importance. We conclude with recommendations for word list creators and researchers and practitioners interested in word importance.