Census relationship files: changes over time versus entity types

Census relationship files answer two different kinds of question. Selecting the right family matters before interpreting a proposed crosswalk. Relationships over time describe the same type of geography across different periods: for example, how places in one census relate to places in an earlier census. Relationships between entity types describe different types within a period, such as congressional districts and places. The Census reference-files guide distinguishes these categories. [S1] A gazetteer is a different starting point: it lists geographic entities and identifying or descriptive information. A tally file supplies counts of entities for a specified area. Neither should automatically be treated as the relationship file needed for a historical join. [S1] Working checklist: write down the source geography, target geography and date for each side of the comparison. Then choose the relationship product that matches that question and inspect its specific documentation. Do not assume that a file described as a crosswalk guarantees a one-to-one mapping or supplies weights appropriate for your statistic. Those checks are editorial guidance, not a claim about every relationship product. Additional source notes For each proposed join, write down the expected cardinality and inspect whether one source code occurs on multiple rows. Preserve those rows until the meaning of their relationship is understood. A one-row-per-code output is a transformation that needs an explicit rule.