Abstract: We survey the evidence bearing on measurement error in the CPI and provide our best estimate of the magnitude of CPI bias. We also identify a "weighting" bias in the CPI that has not been previously discussed in the literature. In total, we estimate that the CPI overstates the change in the cost of living by about 0.6 percentage point per year, with a confidence interval that ranges from 0.1 to 1.2 percentage points. Roughly half of this bias is accounted for by the CPI's inability to fully capture the welfare improvement from quality change and the introduction of new items. Our bias estimate is smaller than that found in several earlier studies, in part because the BLS has recently made a variety of improvements to its procedures; our study highlights several potential areas for further improvement.

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