CPI: revisions, corrections and index families

A CPI observation needs more than a month and a number. Record the index family, population coverage, area, item definition and seasonal-adjustment status alongside the value. These choices identify what is being compared. BLS distinguishes corrections from routine revisions. Its CPI FAQ explains that CPI-U and CPI-W do not need routine revisions for late-arriving reports. Errors can nevertheless require corrections. The chained C-CPI-U is initially published with estimates that are subsequently revised; treating those initial estimates as final loses an important distinction. Seasonal adjustment is another reason to consult the documentation for the exact series. Do not transfer a statement about an unadjusted index to an adjusted series without checking its revision policy. For a reproducible extract, retain the series identifier, observation period, retrieval date and the downloaded file. If two downloads disagree, first compare their series definitions and adjustment flags, then inspect the release notes. A numerical mismatch alone does not identify a methodological break. This note is a guide to the documentation, not a table of index values or a complete history of CPI methodological changes.