Stars and confidence
What the stars mean, and how they are scaled.
What the stars show
The stars show how sure we are about the money-to-bring figure for a country. Each country gets a score from five parts: how good the income data is, the bond yield, inflation, the tax rate and household size. Five stars does not mean certain. Some countries rest on older surveys, and places with no household survey show fewer stars.
How the stars are scaled
We turn the scores into stars by the range of scores, not by fixed cut-offs. Line the countries up by score. The country in the middle of the list gets three stars. The country we are most sure about gets five and the one we are least sure about gets one, with the countries in between placed in proportion on a straight line and rounded to a whole star.
- One very unusual score cannot stretch the scale for everyone: a score far outside the middle half of the list is held at the end of the scale.
- If all the scores sit within 15 points of each other, we do not stretch them into false differences. Each step of 3.75 points from the middle score is then one star.
- Because stars are relative, a change in one country's score can move the stars of other countries even when their own figures did not change.
The money figures never depend on the stars. The stars only describe how much trust the figure deserves.
The method also appears on How it works.