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28.B.1.       Estimation formulas. Fixed (Laspeyres) and current (Paasche) weighted price indices are calculated monthly, quarterly and annually on a customs and balance-of-payments bases for all countries and for theUnited States. The International Trade Division of Statistics Canada also calculates trade values at constant dollars on a balance-of-payments basis through use of the chain Fisher formula with a base reference year. They are available from 1981 to the present on a monthly and a quarterly basis. 

28.B.2.       Error detection and imputation. Once the Laspeyres and Paasche indices have been calculated, a method described by Hidiroglou and Berthelot (1986) to identify outlying observations is used by a module. The Historical Trend Method is also adopted and used to identify transactions within an aggregation that are “abnormal” for a given period. The error detection process is carried out only at the first stage of aggregation in the construction of the International Merchandise Trade Price Index. If a unit value is identified as an outlier during the error detection process, and if the price analyst with the help of the subject-matter specialist also considers this unit value an outlier, then the unit value will be imputed manually. 

28.B.3.       Quality evaluation. The quality of this index is maintained by the few trained analytical experts assigned to this area, who develop a thorough knowledge of the domain. Much time and effort are devoted to detecting and following up unusual fluctuations over time in the pricing patterns of goods. Prior to dissemination, the price indices are analysed and historic trends reviewed.