Access is obtained to a first nonnegative factor matrix and a second nonnegative factor matrix obtained by factorizing a nonnegative asymmetric matrix which represents a set of data which tracks time-stamped activities of a plurality of entities. The first nonnegative factor matrix is representative of initial role membership of the entities, and the second nonnegative factor matrix is representative of initial role activity descriptions. At a given one of the time stamps, while holding a change in the first nonnegative factor matrix constant, a change in the second nonnegative factor matrix is updated to reflect time variance of the set of data at the given one of the time stamps, without accessing actual data values at previous ones of the time stamps. At the given one of the time stamps, while holding a change in the second nonnegative factor matrix constant, a change in the first nonnegative factor matrix is updated, to reflect the time variance of the set of data at the given one of the time stamps, without accessing the actual data values at the previous ones of the time stamps. The role membership of the entities and the role activity descriptions, at the given one of the time stamps, are updated based on the updating steps. A suitable technique for nonnegative symmetric matrices is also provided.