This continual change and diversity contrasts with the simplicity and consistency of Zipf's law, by which the frequency a word, f , is inversely proportional to its rank k, as f ∼ k −γ and Heaps law, by which vocabulary size scales sub-linearly with total number of words, across diverse textual and spoken samples [32, 41, 46, 49, 15, 21, 48,<cite> 42]</cite> .