1. A computer-implemented method of text passage difficulty estimation, comprising: generating, using a computer processing system, an informational text scoring model, wherein generating the informational text scoring model includes: identifying a plurality of texts from a corpus of texts that are informational texts; determining one or more metrics for the informational texts; and configuring the informational text scoring model using the one or more informational text metrics; and generating, using a computer processing system, a literary text scoring model, wherein generating the literary text scoring model includes: identifying a plurality of texts from the corpus of texts that are literary texts; determining one or more metrics for the literary texts, wherein the literary text metrics include one or more metrics that are not included in the informational text metrics; and configuring the literary text scoring model using the one or more literary text metrics, wherein for an input text passage: a set of text variation measures are computed including a text genre metric; the informational text scoring model or the literary text scoring model is selected based on the text genre metric; and a difficulty estimate and a plurality of text variation dimension sub-scores are determined using the selected model; when the difficulty estimate is within an acceptable range of a desired test difficulty level, the input text passage is selected for a test; when the difficulty estimate is outside of the acceptable range of the desired test difficulty level, the input text passage is iteratively adapted using a particular one of a plurality of adaptation strategies prior to being selected for the test, wherein the particular adaptation strategy is selected based on which of the text variation dimension sub-scores is furthest from the desired test difficulty level.