A newly identified source of potential CPI bias: Weekly versus monthly unit value price indexes

•A previously overlooked source of potential bias in the Consumer Price Index is described.•Unit value (average) prices used in the construction of the CPI are the source.•Unit values should be for the same period as the index to be constructed.•Using values for an incomplete sub-period can lead to an upward bias in the CPI.

We find that unit value prices used for constructing the CPI should be for the same period as the index to be constructed, rather than for a sub-period. The latter approach can lead to an upward bias in the CPI.