Airbus and Boeing had used a standard method for measuring component provision level for airlines but it often failed to persuade the low cost carriers to provision the exact level which had recommended. And so many low cost carriers had went ahead to predict their provision level using historical data and carry out some adjustments without using mathematics. They will end up provision less than expected because finance department unable to recognize their justification.
I am glad that I have been in that position to do some contribution to the world. What I proposed was using a simple method learned in college called the inclusion method. By imagining the operation characteristics of AirAsia, and inserting each restrictions (cost, minimum requirement, component turn time, component per aircraft, Airbus/Boeing recommendations, etc) a risk level range from 0 to 3. And multiply them and get an optimum level of required asset, specially tailored for AirAsia alone. For example, expensive items can buy less while cheap items can buy exact item recommended. Too expensive items can buy 1 since it might exceeded the cost of delay vice versa.
This method was published to Airbus and recognized during Reliability symposium in Toulouse. I think will save operators millions of investment but maybe its bad news for the OEMs. LOL.