How does a three-digit number decide whether you get a loan?

A credit score is a number that sums up how likely you are to repay. It is built from your credit files, and lenders use it to decide who qualifies for a loan, at what interest rate, and with what credit limit. In the United States, one version is the FICO score, created by a company founded in 1956 by engineer Bill Fair and mathematician Earl Isaac. Base FICO scores run from 300 to 850.

What goes in? In the US, the score is mostly based on reports from the three big credit bureaus, Experian, TransUnion and Equifax, and income and employment history are not counted. The FICO recipe gives payment history the biggest weight, 35 percent, then debt burden at 30 percent, length of credit history at 15 percent, and 10 percent each for the types of credit used and for recent searches for credit.

But other countries do it differently. In the United Kingdom there is no universal credit score: each lender assesses borrowers on its own criteria, and those algorithms are effectively trade secrets. In Germany, a private company called Schufa provides scores for about three-quarters of the population.

France takes yet another route. Its national file, the FICP, run by the Banque de France, lists people who were unable to repay a loan on time. It is a negative file, recording payment incidents rather than all of someone's loans. A proposal for a French positive file listing every loan was rejected by the Constitutional Council in March 2014, which judged the method disproportionate given its effect on privacy.

Credit history also usually stays within one country, so someone who moves can arrive with an excellent record that the new country's lenders cannot see.