What these comparisons measure
A cost of living comparison answers one question: how much money you need in one place to buy what a given sum buys in another. It does not measure salaries, taxes or quality of life. It measures prices, and only the prices of an average household basket.
The figures here come from purchasing power parities published by statistical offices. A parity says how much of a country's own currency it takes to buy one standard basket. Comparing two parities gives a price ratio between the two places, and because both sides are prices in their own money, that ratio is free of exchange rates. A euro-dollar move does not change the answer.
Why the numbers are not from a crowd-sourced site
The best known cost of living sites collect prices from visitors who volunteer them. That produces a lot of coverage and no published method, no sample control and no way for anyone to check a figure. Every number on this site comes from a statistical office that publishes both the figure and how it was produced, and every page names the document it came from. Where official data stops, these pages stop too, and say so.
What official data does not cover
Inside the United States, price levels are published for every state and for hundreds of metropolitan areas, so a comparison can be quite local. In Europe they are published for whole countries only. There is no annual official series for Berlin against Munich, or Lyon against Paris, so this site does not offer one. A country figure is a national average, and a capital city usually sits above it.
The basket is also an average household's, not yours. Housing is the largest single reason an individual's experience departs from the index: a household that owns outright and one paying central city rent face very different price levels in the same city. Where the source publishes a housing component separately, the breakdown table shows it.
Reading a comparison next to a salary
Price levels answer half of a relocation question. The other half is what each tax system leaves you, which the take-home comparison covers. A place can be twenty per cent cheaper and still leave you worse off if it withholds far more of your gross, and the reverse happens just as often. Each comparison here links to the matching salary comparison for the same two places.