Change in average inflation rate measures if a country's inflation rate got better or worse during a 4-year presidential term.

Change in 4-year average inflation rate = (average monthly inflation rate from the start (January) of the current 4-year term to the month with the most recent inflation data before election day) - (average monthly inflation rate from the start (January) of the previous 4-year term to the month before the current term's inauguration (December))

For the last 18 presidential elections - since 1956, Change in Average Inflation Rate has correctly predicted 17/18 popular vote wins. In every presidential election since 1956 except 1972, whenever the average monthly inflation rate goes up, the incumbent party loses the popular vote.

3.20% = 5.10% - 1.90%

*The data in the "Avg up to Election" column uses the average of the months up till the September of the election year. This will likely be the most recent month that has data during the 2028 election.

The average monthly inflation rate during Joe Biden's presidency was 4.95%. In order for the Incumbent (Republican) Party to be predicted as the 2028 popular vote winner, the average monthly inflation rate from January 2025 to the 2028 election would need to be below that.

The data will be retrieved from https://www.rateinflation.com/inflation-rate/usa-historical-inflation-rate/, https://www.usinflationcalculator.com/inflation/historical-inflation-rates/, or a similar source that is available.

The metric appears to only be effective for elections during 1956 and later. Between 1956 and 2024 its accuracy rate is 17/18, but between 1924 and 1952 it is only 2/8. This could possibly be because of the significant events of that period (WW1, The Great Depression, WW2) having substantial and erratic effects on voting reasoning and inflation.