1. A method for a Radio Frequency Identification (RFID) system, comprising: inventorying an RFID tag to obtain an item identifier (II) indicating an item to which the tag is attached; determining at least one read metric, wherein the at least one read metric is determined from at least one of a tag response and a reader state; retrieving, based on the II, a read difficulty factor (RDF), wherein the RDF indicates how the item modifies a readability of the tag and was stored prior to the inventorying; and estimating a parameter based on the at least one read metric and the RDF, wherein the parameter is at least one of a tag velocity, a tag motion direction, and a tag distance from a reader antenna, and estimating the parameter includes updating the at least one read metric based on the RDF.