Shifting Ground: How Federal Agencies Quietly Redraw Their Statistical Baselines — and Why Your Longitudinal Analysis May Already Be Broken
When the Census Bureau redefines a survey population, the Bureau of Labor Statistics revises its reference period, or the CDC quietly updates its weighting methodology, the time-series data that researchers depend on develops invisible fractures. Most published studies never account for these structural discontinuities, yet the downstream consequences for longitudinal analysis and policy modeling can be severe. This article examines documented instances of baseline volatility across major federa