Estimating the size of hidden data sources by queries

The sizes of hidden data sources are of great interests to public, researchers and even business competitors. Estimating the size of hidden data sources has been a challenging problem. Most existing methods are derived from the classic capture-recapture methods. Another approach is based on a large query pool. This method is not accurate due to the large variance of the document frequencies of queries in the query pool. Targeting this problem, we propose a new method to reduce the variance by constructing a query pool from a sample of the target data source so that document frequency variance is reduced, yet most of the documents can be covered. Our method is tested on a variety of large textual corpora, and outperforms the baseline random query method and the Broder et al´s estimation method on all the datasets.