The method that is used in NPRD-2011 to determine the component failure rates is described in Section 1 of that publication. Failure rate calculations performed on only zero failure data records are accomplished by summing the total operating hours, calculating a failure rate by assuming one failure, and denoting the resulting worst case failure rate with a “<” sign. In the example that you provide, the total operating time is 7368 hours (0.002456E6)x3, and the total number of failures is 0. Therefore, the merged failure rate should be < 135.722 failures per million hours. For data records containing failures, the equation found on page 1-15 of NPRD-2011 is used to calculate the failure rate.