How to calculate the No Fault Found Ratio for a Line Repairable Unit.
Background
Technicians often remove Line Repairable Units (LRUs) when there is nothing wrong with them.
If they send the LRU back to the service centre and it is diagnosed as serviceable this is classified as a No Fault Found (NFF).
NFFs are important in Spare Parts forecasting because if a unit is away for repair then the effect on the logistic system is identical to a true failure.
Therefore, from a Spare Parts perspective the NFF ratio and MTBF are equally important.
Mathematics
In mathematics ratios are expressed in the format of A to B or (A:B).
Therefore, NFF Ratios can be expressed as No Fault Found : Fault Found or (NFF:FF)
Example 1
Consider the case where we receive 4 units for repair and 3 of the units are diagnosed as NFF and 1 unit is diagnosed as a FF.
We can therefore express the NFF ratio as 3:1, or 300%.
Example 2
Consider the case where we receive 4 units for repair and 1 unit is diagnosed as NFF and 3 units are diagnosed as FF.
We can therefore express the NFF ratio as 1:3, or 33.33%.