A method of determining an estimate of the market value of a traded unit of a financial instrument, and apparatus for carrying out the method, said instrument comprising a fund of individually priced securities and the exact composition of said fund being withheld from the market, said method comprising selecting a plurality of mutually independent risk factors, each risk factor being representative of market behaviour estimated to be significant to the price behaviour of the traded unit, obtaining information from a third party holding information regarding the composition of said fund regarding the actual significance of said risk factors to the value of said traded unit, and calculating a value for said traded unit on the basis of said significances.