Measuring lexical diversity in texts that have different lengths is problematic because length has a significant effect on the number of types a text contains, thus hampering any comparison. Treffers-Daller et al. (2018) recommended a simple solution, namely counting the number of types in a section of a given length that was extracted from the middle of each of the texts to be analysed. By applying this approach to second language essays, the authors observed that using the number of types was slightly more effective for differentiating amongst the Common European Framework of Reference for Languages (CEFR) levels than were the Type-Token Ratio (TTR) and the Guiraud index and that these three indices were more effective than were mathematically more complex ones. However, their conclusions regarding these two points are incorrect, and a less basic approach should be used. The last section addresses two broader issues in applied linguistics that are highlighted by these problems.