calculating is difficult because the productivity can be changed from time to time due to unforeseen有冇qs contractor 可以大慈大悲俾我問下功課 感激不盡 條問題係關於contractor 會用咩method 去measure 單work 既productivity tg:WalH2OterMostly, empirical judgement based on previous project experience or just ask the subcontractor.
site conditions, construction difficulties, management skills of contractor