The Gross National Product (GNP) deflator is a measure used to adjust the nominal GNP for changes in the overall price level of goods and services in an economy. It is a price index that reflects the average price change of all goods and services produced by the residents of a country, whether produced domestically or abroad.

The GNP deflator is calculated using the formula:

\[ \text{GNP Deflator} = \left( \frac{\text{Nominal GNP}}{\text{Real GNP}} \right) \times 100 \]

Here:
– **Nominal GNP:** The total value of goods and services produced by a country’s residents, measured at current market prices.
– **Real GNP:** The total value of goods and services produced by a country’s residents, adjusted for changes in the price level (inflation or deflation).

The GNP deflator is expressed as an index number, usually normalized to a base year set at 100. An increase in the GNP deflator over time indicates inflation, while a decrease indicates deflation.

Key points about the GNP deflator:

1. **Inflation Measurement:** The GNP deflator serves as a broad indicator of inflation within a country. By comparing nominal GNP to real GNP, it captures the impact of changing prices on the overall economic output.

2. **Nominal vs. Real GNP:** Nominal GNP reflects the current market value of goods and services without adjusting for changes in prices. Real GNP, on the other hand, adjusts for inflation or deflation, providing a measure of the actual physical output.

3. **Base Year:** The GNP deflator is often referenced to a specific base year, and changes in the index are relative to that base year. This allows for a standardized comparison of price changes over time.

4. **Economic Growth Adjusted for Inflation:** When evaluating economic growth, it’s common to use real GNP rather than nominal GNP. The GNP deflator helps analysts and policymakers understand how much of the observed change in nominal GNP is due to actual economic growth and how much is due to changes in prices.

5. **Comparison Across Years:** An increase in the GNP deflator indicates that prices have risen, and the nominal GNP has increased more than the real GNP. Conversely, a decrease suggests deflation, where prices have fallen.

6. **Policy Implications:** Central banks and policymakers use inflation measures like the GNP deflator to inform monetary policy decisions. It helps them gauge the overall price stability within an economy.

While the GNP deflator is a useful tool for measuring inflation in an economy, other price indices, such as the Consumer Price Index (CPI) and the Producer Price Index (PPI), may be used to provide more specific information on changes in prices for consumer goods or at the producer level.