DONNOKDoes a new CPI base year change your purchasing power?
A CPI of 100 is a reference point. A worked example shows why rescaling an index changes its numbers but preserves purchasing power ratios.
What does an index of 120 mean?
With the reference price level set to 100, an index of 120 means average prices are 20% above that reference. It does not mean every product or your own living costs rose exactly 20%. The figures below are hypothetical.
Rescale 100 and 120 together
A starting index of 100 and ending index of 120 give a purchasing power factor of 100 ÷ 120 = 0.833333…. Set the endpoint to a new base of 100 and the start becomes 83.333333…. The ratio stays the same. An unchanged amount of 1,000 has about 833.33 of purchasing power at starting prices under either scale. The table rounds the values.
| Reference scale | Start index → end index |
|---|---|
| Start = 100 | 100 → 120 |
| End = 100 | About 83.3333 → 100 |
| Start ÷ end | About 0.833333 on both scales |
Do not divide numbers with different bases
Taking an old-base start of 100 and a new-base endpoint of 100 would wrongly imply no change. Use a consistent series with the same country, measure, frequency and base. A statistical update that also changes items, weights or past data can differ from simple rescaling.
Check the data basis in DONNOK
DONNOK’s personal display estimates purchasing power with Korean CPI. Check its methodology and data status; changing currency does not change the CPI country. This example explains index ratios, not the app’s complete daily interpolation or update process.