Theoretical Explanation of Observed Decreasing Failure Rate
Technometrics, 42 (1):
7--11 (February 2000)Abstract
Pooled data on the times of successive failures of the air conditioning
system of a fleet of jet airplanes seemed to indicate that the life
distribution had a decreasing failure rate. More refined analysis
showed that the failure distribution for each airplane separately
was exponential, but with a different failure rate. Using the theorem
that a mixture of distributions each having a non-increasing failure
rate (e. g., a mixture of exponential distributions) itself has a
non-increasing failure rate, the apparent decreasing failure rate
of the pooled air-conditioning life distribution was satisfactorily
explained. The present study has implications in other areas where
an observed decreasing failure rate may well be the result of mixing
exponential distributions having different parameters.
