15.1. The present chapter focuses on good practices in the collection, validation and reporting of quantity information based on the recommendations contained in chapter V, on quantity measurement, of IMTS 2010. It provides details on the World Customs Organization (WCO) standard units of quantity and elaborates the concept of net weight. The chapter covers the subject of data compilation from customs and non customs sources, the conversion of non-standard quantity units to the WCO standard units and net weight, quality issues and the estimation and imputation of missing quantities. The annexes to the chapter present mathematical conversion factors and conversion factors applied by UNSD. This chapter is linked to chapter XIII on the Harmonized Commodity Description and Coding System (HS), as the standard units of quantity are specified in terms and as part of the HS. Section D on quality issues, which is linked to Chapter IX, contains additional information regarding the quality assurance of quantity and net weight.