For this model to make sense, h needs to be an S-shaped curve so that the estimated probability of a reader knowing a word increases monotonically as the difference between the annotator's level (denoted L a ) and the word's complexity (denoted C w ) increases (see Figure 6).
we choose to define h as follows: where a is the annotator, w is the word, and φ is the sigmoid function φ(x) = 1/(1 + e −x ).