Evidence and data
Use money aggregates, velocity, nominal GDP, and inflation. Velocity instability is the first diagnostic check.
Calibration or measurement
Velocity behavior and real-output assumptions determine whether money growth maps into inflation.
Boundaries
- Short-run velocity can move sharply.
- Credit and endogenous money complicate the stock measure.
- Causality cannot be inferred from the identity alone.
Use guidance
- When sufficient
- Very long-run cross-country comparisons of price levels when velocity is stable and the monetary regime is well-anchored. Countries with persistent double-digit money growth tend to have persistent double-digit inflation; the identity MV=PY is a reliable organizing frame at decade-length horizons where real output growth and velocity drift are relatively predictable.
- When sketch only
- Do not use for short-run inflation forecasting under modern central-bank operating frameworks, where the relevant policy instrument is the interest rate and the relevant money aggregate is endogenous. Velocity instability and the shift to abundance-reserves regimes have broken the short-run M-to-P mapping in most advanced economies.
- When to switch
- Switch to the New Keynesian Phillips curve (theoretical:new-keynesian-phillips) for short-run inflation dynamics. Switch to a velocity-adjusted money-demand model or a term-structure model for medium-run analysis where the distinction between base money and broad money matters.
- Falsification signal
- The post-2008 expansion of central-bank balance sheets in the US, UK, EU, and Japan produced large increases in the monetary base without corresponding increases in inflation or broad money. A chart showing base money and CPI inflation diverging for five or more years after a large QE program constitutes a clear falsification of the naive quantity-theory prediction for that policy horizon.