Consequently, differences of several tenths of a percentage point may occur between rates of change computed from the BLS published index numbers compared to those computed from the BLS source data. BLS measures of productivity and costs are based on underlying series from a variety of sources. These data sources are frequently revised as additional data become available. When any of these underlying series is updated or revised, the productivity and costs measures are revised to reflect the new information. Generally, a recent quarter of data is revised twice after its initial release.
Historical revisions occur when the source data used in their construction are revised.
Because many of these sources are revised independently, the productivity measures undergo frequent revision.