15. The method of claim 1 wherein text genre metric is computed using factors from the group consisting of usage rates of particular verbs, frequencies of first and second person pronouns, frequency of question marks, and ratio of nouns to verbs.
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.