An increasingly used instrument by central banks, which consists in asking a representative sample of the population to record for a certain period of time all their transactions, as well as the See Payment instrument. used. A transfer of funds which discharges an obligation on the part of a payer vis-à-vis a payee. diaries provide a snapshot of the use of different payment instruments. They are, however, a costly tool and do not provide real-time results, but are useful for comparative purposes.