In this paper we explore various methods to estimate the collection parameter of the information based models for ad hoc information retrieval. In previous studies, this parameter was set to the average number of documents where the word under consideration appears. We introduce here a fully formalized estimation method for both the log-logistic and the smoothed power law models that leads to improved versions of these models in IR. Furthermore, we show that the previous setting of the collection parameter of the log-logistic model is a special case of the estimated value proposed here.