Experiments on predictability of word in context and information rate in natural language

Based on data from a large-scale experiment with human subjects, we conclude that the logarithm of probability to guess a word in context (unpredictability) depends linearly on the word length. This result holds both for poetry and prose, even though with prose, the subjects don't know the length of the omitted word. We hypothesize that this effect reflects a tendency of natural language to have an even information rate.