Validity vs reliability distinction in GDP measurement

GDP data should be assessed on two separate dimensions: 'validity' (whether the level estimate is correct, which in rich countries is off only a few percent but in sub-Saharan Africa can be plus-or-minus 50-100%) and 'reliability' (whether the methodology stays consistent over time); the bathroom-scale analogy shows that a consistently miscalibrated scale is fine for tracking change, but a scale someone secretly recalibrates in the night ruins time-series comparison, while comparing yourself to a neighbor using a different scale ruins cross-country comparison.