...Then all the lemmas are queried in the Arabic Gigaword corpus (fourth edition) and if a lemma has a frequency of 10 or less occurrences, then it is considered as obsolete.
Reference
Mohammed Attia, Pavel Pecina, Lamia Tounsi, Antonio Toral, Josef van Genabith. 2011. A Lexical Database for Modern Standard Arabic Interoperable with a Finite State Morphological Transducer.