Economists love to compile indices aggregating lots of individual data, so they can analyze broad trends in the behavior of an economy. Inflation is measured by an index of consumer (retail) prices. There are indices of all sorts of things that are bought and sold of which perhaps the best known are share price indices like the Dow Jones Industrial Average or FTSE-100. The main challenges in compiling an index are what, exactly, to include in it and what weight to give the different things that are included. A particularly tricky question is how to change an index over time. Measures of inflation are based on the price of a basket of things bought by a typical consumer. As the quality and choice of products in the basket change over time, the inflation index ought to take this into account. How, exactly, is much debated.