Deconstructing the Library of Babel: Analysis of linguistic complexity in Spanish graded readers and literary works

Graded readers (GRs) are a popular language-learning resource, as they provide contextualized input adapted to any level. Still, their creation process is non-systematic and their quantity is limited. This article investigates, firstly, the progression of linguistic complexity in a series of Spanish GRs of consecutive levels, and secondly, whether literary works (LWs) targeted at specific age groups of L1 speakers exhibit a similar gradation, thus representing suitable complementary material. For this purpose, we (1) fitted two random forests on 40 complexity features computed by processing 50 GRs, 50 LWs, and 8,585 graded lexical items; (2) performed intergroup comparisons with a reference corpus through permutation tests on the four features most informative to the random forests; and (3) further explored vocabulary using distributional techniques. Our findings indicate that complexity does not progress uniformly across levels or linguistic dimensions for both GRs and LWs. Moreover, LWs only differ substantially from GRs in their lowest level. This demonstrates the importance of considering quantitative measures such as the ones presented in this study to develop balanced language teaching materials.