Harold (Harry) E. Ascher was known widely in the reliability engineering community for his many contributions, especially in the area of statistical modeling of repairable systems reliability.
The 1984 book co-authored by Harry Ascher and Harry Feingold, Repairable Systems Reliability: Modeling, Inference, Misconceptions and Their Causes [1] was the first book to thoroughly address statistical modeling of systems that are repaired rather than discarded after the first failure.
Harry Ascher earned degrees in Operations Research from City College of New York and New York University and spent most of his career with the Naval Research Laboratory (NRL) in Washington DC. Following his retirement from NRL, he spent more than 20 years as a private consultant.
He received many distinctions and awards including the Allan Chop Award from ASQC (now ASQ) in 1995.
One of Harry Ascher’s greatest visions, as a professional, was to “clean up” notation and terminology from what he considered to be results of inherent and avoidable ambiguities between properties of repairable systems and, in contrast, properties of (non-repairable) parts.
For example, the term “Failure Rate” in the literature is used in multiple conflicting meanings, sometimes even within the same article.
One meaning is the derivative of the expected cumulative number of failures for a repairable system (also known as a “Rate of Occurrence of Failures” of “Failure Intensity Function”).
The other, quite different, meaning is the ratio of the Probability Density Function and the Reliability Function for a part (also known as the “Force of Mortality” or “Hazard Function”).
He introduced us the fact that the most important analysis to perform on statistical data for a repairable system is to test for trend.
This may possibly classify the system as a “Happy System” (times between failures tend to get larger and larger, a “Sad System” (times between failures tend to get shorter and shorter) or a “Non-Committal System” (times between failures show no significant sign of trend).
He would repeatedly state that, when it comes to modeling a repairable system, “a set of numbers is not a data set.” Often, the particular pattern in which failures occur provide more useful information about
a system, than does a set of numbers describing the observed times between failures.
He taught us that a renewal process is generally an inappropriate model to use for a complex repairable system consisting of many parts that have either “infant mortality” or “wear-out” characteristics.
To demonstrate how absurd the renewal process model is, when used to describe a complex system such as a car he often told his “Honest John” story.
This is about a used-car salesman trying to sell a wornout car with multiple mechanical issues by telling the customer: “Two days ago the battery was dead, so we charged it. So the car is two days old!”
(excerpted from article by Christian K. Hansen, President, IEEE Reliability Society)
Previously published in the June 2014 Volume 5, Issue 2 ASQ Reliability Division Newsletter
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