the rest to build the full distance matrix. 226 227 We tested our algorithm on datasets from the two most recent PAN CLEF 228 (2013) competitions, which provide benchmark datasets for authorship attri- 229 bution. From PAN 2013 we selected the author identification task described 230 in Juola and Stamatatos (2013). In this task 349 training texts are provided, 231 divided in 85 problems out of which 30 are in English, 30 in Greek and 25 232 in Spanish. For each set of documents written by a single author it must be 233 determined if a questioned document was written by the same author or not. 234 Each text is approximately 1000 words long, which is close to our empirical 235 15 3.3. The PAN Benchmark Dataset Table 2: Classification results on the Liber Liber dataset. Each text from the 11 au- thors is used to query the database, and it is considered to be written by the author of the most similar retrieved work. The authors’ full names: Dante Alighieri, Gabriele D’Annunzio, Grazia Deledda, Antonio Fogazzaro, Francesco Guicciardini, Niccol´ o Machi- avelli, Alessandro Manzoni, Luigi Pirandello, Emilio Salgari, Italo Svevo, Giovanni Verga. The CNG method has been tested using the reported amounts of n-grams. Author Texts FCD CNG-500 CNG-1000 CNG-1500 Dante Alighieri 8 8 6 5 7 D’Annunzio 4 4 4 3 4 Deledda 15 15 15 15 14 Fogazzaro 5 5 4 5 5 Guicciardini 6 6 5 5 5 Machiavelli 12 10 8 10 9 Manzoni 4 4 4 4 4 Pirandello 11 11 5 10 8 Salgari 11 11 10 10 9 Svevo 5 5 4 5 5 Verga 9 9 6 9 8 Total 90 88 71 81 78 Accuracy (%) 100 97.8 78.9 90 86.7