In this article, the encounter probability, possibility to find unknown words in free descriptions was proposed. When a researcher thinks the amount of data was sufficient and decides to stop collecting data, this index gives us useful information. By the encounter probability, we can know how often we get unknown words. Free description data from two companies was analyzed by some Pareto distributions to decide most suitable model to explain frequency of words. Parameters of these distributions were estimated by using Markov chain monte carlo mehod. Consequently it was shown that these free descriptions were sufficient amount of information by calculating encounter probability.