In (Corney, 2003;Gamon, 2004;Peng et al., 2004;Koppel and Schler, 2004) the datasets were rather small in terms of the number of authors, indicating the results may not be generalisable.
in (Corney, 2003) the largest dataset contains texts from five authors, in (Gamon, 2004) from three, while in (Peng et al., 2004) and (Koppel and Schler, 2004) from ten.