As for log normal repair time, two factors have to be considered:

  • the shape of log normal curve is asymmetric;
  • it is skewed to the right. Thus, the centre or mode being to the right most of the repair times will be distributed around this centre with relatively few longer repair times in the right hand tail of the distribution. This is related to the MTTR.
Example: An electronic assembly (or a chemical instrumentation scheme a fuel pump or a ball mill drive system, an avionic panel, etc.) is required to be repaired in 3 hr. This is desirable in cases of about 90%. If the repair distribution is log normal with s=0.45, what is the goal MTTR (designed) to be achieved?