Which exchange rate should I use for calculating Bitcoin exchange rates? I'm using the Blockchain exchange rate API to add a Bitcoin exchange rate calculating function to one of my apps.  I'm not sure which value I should be using.  For each currency type (USD, EUR, etc.) four different rates are returned: "last," "buy," "sell," and "24h."
Most people are interested in the last value or the 24 hour average, it depends on the purpose of the application and the intent of the developer.