All Change! The Implications of Non-Stationarity for Empirical Modelling, Forecasting and Policy
Bibliographic record
Abstract
Economies, societies, and many natural systems evolve and change, sometimes dramatically, so good models and accurate forecasts are vital for policymakers to prepare for and navigate these changes successfully. Yet history is littered with forecasts that went badly wrong, sharply illustrated during the recent recession. A glance at most economic and related time series, such as greenhouse gases, reveals the invalidity of an assumption of stationarity, whereby the mean and variance are constant over time. Nevertheless, many models used in empirical research, forecasting or for guiding policy have been predicated on treating observed data as stationary, when in fact such analysis must take non-stationarity into account if it is to deliver useful outcomes. The problem for policymakers is not a plethora of excellent models from which to choose, but to find stable relationships that survive long enough to be useful. This paper offers guidance for policymakers and researchers on identifying what forms of non-stationarity are prevalent, what hazards each form implies for empirical modelling and forecasting, and for any resulting policy decisions, and what tools are available to overcome such hazards.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.021 | 0.126 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.004 |
| Science and technology studies | 0.002 | 0.007 |
| Scholarly communication | 0.006 | 0.013 |
| Open science | 0.002 | 0.003 |
| Research integrity | 0.004 | 0.007 |
| Insufficient payload (model declined to judge) | 0.005 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".