Over 20 years ago the Assistant Secretary of the Army directed the Army to not use MIL HBK 217 in a request for proposals, even for guidance. Exceptions, by waiver only.
217 is still around and routinely called out. That is a lot of waivers.
Why is 217 and other parts count database prediction packages still in use? Let’s explore the memo a bit more, plus ponder what is maintaining the popularity of 217 and ilk.
The 1995 Decker Memo
Gilbert F. Decker signed a short policy statement in a memorandum. The subject is “Policy on Incorporating a Performance-Based Approach to Reliability in Request for Proposals (RFPs)”. He states that reliability requirements should include:
1) quantified reliability requirements and allowable uncertainties.
Reliability as a metric is a quantifiable requirement and includes the probability of survival, duration, environment, and function. I do not agree that a requirement or goal should include ‘allowable uncertainties’ as the requirement is the desired state for the population. Although using confidence levels for sampling is common practice as testing 100% of units to failure is not practicable.
2) failure definitions and thresholds
This is part of the reliability metric, what is the function, and when that doesn’t work, that is a failure.
3) life-cycle conditions
Again already part of the reliability metric.
The memo goes on to include a rationale that including failure definitions and life-cycle conditions are necessary for a fully defined reliability requirement. Duh! See the definition of reliability and it’s essential four elements.
It seems the memo has missed the duration element of the reliability metric – that is unfortunate.
MIL HDBK 217 and the Memo
My favorite part of the memo and why a good friend sent the memo to me was the specifically stating that RFPs should not use 217 at all. And should not request the use of parts count predictions in general.
While there are plenty of reasons to avoid 217 and similar, the memo states they have been ‘shown to be unreliable and its use can lead to erroneous and misleading reliability prediction.’
Even though the 217 document itself says essentially the same, not to be used to predict field performance, the Assistant Secretary of the Army felt the point needed reiterating.
Why does 217 Remain in Use?
The easy answer is there are plenty of non-thinking people in the world that want to just what something easy to ‘get reliability’ done.
The harder answer is we all want to predict the future reliability performance for our products. We want to know what will fail and when. That is difficult to do well. It costs money, time, resources, and is still difficult to get accurate results.
Doing what has been done before, predictions, what is easy, parts count predictions, just seems too compelling for too many. Thus 217 remains part of RFPs and too many reliability programs.
What Can We Do Today?
Well to start, start using performance-based reliability requirements. When setting requirements use reliability (A function or set of functions that should survive a duration(s) with some probability given a set of environmental conditions.) It is complete and performance-based.
When confronted with the results of a prediction, especially a parts count or 217 based prediction, refrain from laughing as you toss out (delete) the bit of worthless information. Instead, restate the request for meaningful information concerning what will fail and when.
Physical of failure modelings, accelerated life testing, and other approaches provide a means to estimate future performance. Tallying up failure rates does not.
Now if only we can find (or create) memo’s outlawing the use of MTBF!