<prompt> Translate the following text to EnglishKiasi cha vipengele ni idadi ya vipengele katika jamii na bei ya vipengele ni fasta uzito viashiria ya mbalimbali ya bei kulipwa na mailpieces kuunda jamii.  <response>: The volume variables are the number of pieces in the category and the price variables are fixed-weight indexes of the range of prices paid by the mailpieces making up the category. 