Millions of adults in the United States lack the necessary literacy skills for most living wage jobs. For students from adult learning classes, we used a lexical decision task to measure their knowledge of words and we used a decision-making model (Ratcliff's, 1978, diffusion model) to abstract the mechanisms underlying their performance from their RTs and accuracy. We also collected scores for each participant on standardized IQ tests and standardized reading tests used commonly in the education literature. We found significant correlations between the model's estimates of the strengths with which words are represented in memory and scores for some of the standardized tests but not others. The findings point to the feasibility and utility of combining a test of word knowledge, lexical decision, that is well-established in psycholinguistic research, a decision-making model that supplies information about underlying mechanisms, and standardized tests. The goal for future research is to use this combination of approaches to understand better how basic processes relate to standardized tests with the eventual aim of understanding what these tests are measuring and what the specific difficulties are for individual, low-literacy adults. [This article was published in "Cognition," v146 p453-469 Jan 2016.]