Category Archives: MTBF

Mean Time Between Failures or MTBF is a common metric for reliability and is often misused or misunderstood.

Book and Course projects

MTBF

Traffic

Over the past two weeks this site has received over 150 visitors each weekday. From what I can see in the analytics and from a few conversations with folks, the site provides insights and information around the use of MTBF, plus basic information concerning reliability engineering.

Google tends to like the site as they agree that visitors like the site, too.

Book project in search of feedback

Given the interest and plenty of encouragement (and helpful suggestions) I’m putting together a book based on the NoMTBF material. Not just bashing MTBF, although there is plenty of that, but also the steps to use reliability or other measure that provide better information.

I have the basic outline and draft completed and am now ready for some feedback. If you’d like to review the work, conditional on you providing you feedback, suggestions, ideas and comments, let me know and I’ll send you a draft copy.

The draft needs work on formatting, layout, adding clean graphics, etc. Yet the outline and basic text is there.

Can you follow the argument, is the writing clear, is there anything missing, how about the order or emphasis?

It’s not a long work, right now about 22,000 words or depending on book page size, fonts size, margins, etc. about 100 to 120 pages. In word it has 73 pages right now without any attention to formatting.

If you have the time and interest let me know and I’ll send you copy, but you have to comment, critic, and make suggestions. I really would like this work to be useful for you and for use to encourage others to avoid using MTBF.

Course project in search of ideas and direction

This period of reflection concerning the NoMTBF project has reinforced the idea that we need to provide something concrete and positive to do instead of just not doing MTBF. Part of the issue is our education system, standards, and textbooks as they often include MTBF in examples and at length in the discussion.

So, the idea is to create a course for experienced reliability professionals and interested engineers and managers with an interest in reliability, that focuses on reliability metrics from goal setting to tracking performance.

I’ve the technology to put together an online course that could be self paced or provided on a fixed schedule (say weekly). It could include short lectures, discussions, reading material and quizzes or examples to work.

Here’s a draft outline – what do you think?

  1. Reliability definition and how it is used in engineering decision making

  2. Common reliability measures: pros and cons

  3. Reliability and Availability Goal setting – connecting the goal to your business objectives

  4. Estimating reliability for comparison to the goals

  5. Tracking reliability and reporting performance

  6. Reliability testing with results that compare to goals

  7. Reliability modeling that leads to meaningful discussions and decisions

  8. Common mistakes and remedies concerning reliability measures

  9. How to get useful reliability information from vendors

(plenty of opportunity for bashing MTBF, yet if done in contrast to much better methods and measures, may provide really practical and useful information.)

So, thoughts? What would you want added, emphasized, and what would you want to be main take aways for each topics? What would you like to see in the course for yourself or for those you’d recommend take the course?

If you’d like to participate in the course project, I’m very open to your ideas and suggestions. Maybe help create and present a topic, provide examples, or sample problems or discussion questions.

Anyway, looking for feedback and ideas to make the NoMTBF site much more positive and useful for the reliability engineering community and for anyone interested in reliability.

Well thought out feedback

MTBF

A note from Scott – providing feedback on the NoMTBF site.

Hi Fred,

Your website has generated quite a bit of valid conversation about MTBF. I applaud you for that. Honestly though I have mixed feelings about some of what you present and thought I’d write this lengthy e-mail to provide some feedback. I hope you take this in the right light as constructive criticism from someone who, overall, appreciate your efforts.

Clarifications

Let me start with a point I disagree with. In your opening slide show “Thinking about MTBF” I think the “Common Confusion” slide could be better presented. Many viewers would interpret that slide to say that the MTBF is not the mean. Of course MTBF is the mean. Your point is that, while it is the mean, the distribution is not Gaussian. Fair enough. Funny thing is I’ve actually had quality engineers try and tell me the MTBF is not the mean of the distribution and I’m afraid your slide may perpetuate that misunderstanding.

In the same vein, later in the talk, and in the other sections on your site, you seem to indicate that the MTBF is not the expected value (See Perils “I heard one design team manager explain MTBF as the time to expect from one failure to the next.”). Of course the MTBF is the expected value. That is from a pure mathematical sense (as you discuss earlier in this section). So I’m confused on your point here. I guess you are commenting on the laymen’s feeling for “expected” value. Which leads me to my next section.

Lack Of Understanding of Statistics

It almost appears that one of the premises of NoMTBF is that many people do not understand statistics and therefore we should not confuse them by using MTBF. I disagree with this. For example, many people don’t understand the difference between median and mean but no one is suggesting we remove those terms. Similarly because many people incorrectly assume a Gaussian distribution when they hear the term mean is hardly justification for removing the term MTBF. The problem is education not the definition. Same point for expectation. Because the average is some value does not imply all samples will be equal to that value. Anyone who thinks that, in my opinion needs more education in statistic and we shouldn’t try and “simplify” to account for lack of education.

Constant Failure Rate

I don’t really accept your implication that using MTBF implies constant failure rate. The proper definition is the integral form you present in a number of spots but I agree that many tie these two together. I think one of the themes of your website is that the constant failure rate assumption is not valid. In that, I’m in 100% agreement and applaud your efforts. (I guess the site name would not have the same panache if it was called NoConstantFailureRate). Clearly the constant failure rate model often does not apply and reducing all of reliability to one number is a gross simplification.

Leadership

So where should people go instead? Just bashing something is not a solution. Your website really has had an impact but in a strange way sometimes it has had the opposite impact than what I think we would both like. I’ve had quality managers who did not want to gather the data on field failure with, in part, the justification that MTBF is bogus statistics. OK MTBF is not perfect but I’m sure we agree that the way to improve reliability is to gather data as a first step.

You have quite a following and, personally, I’d like to see you to lead more. Yes MTBF is a simplification but I also don’t expect to pick up a data sheet and see physics of failure paper stabled to the back of it or a chart of reliability over time. Fact is many complex things get reduced to a few key numbers (e.g. horsepower, MPG, 0 to 60 time for a car). I think your Actions/Alternative Metric is addressing this. Stating a reliability percentage over a time interval is an intriguing alternative. I like it. If that is your alternative then, personally I’d like to see it more clearly emphasized across the site. I’d also like to see you develop it more. How does one determine reliability % and duration from the Weibull parameters? How would one put together a reliability block diagram and estimate overall reliability if subcomponents were specified in this manner? I don’t know that answer to these questions and I’d be interested in reading more.

As I stated in the beginning, I hope you take this in the right light. While obviously I don’t agree with everything on your site you have many extremely valid points and you are doing a great job stimulating discussion. Thanks for your efforts.

Scott Diamond
Vice President of Quality and Customer Excellence
Surveillance Group
FLIR Systems Inc.

 

— Ed note:

Thanks Scott for the insightful and meaningful feedback – I will be making some adjustments and improvements. Thanks for the careful reading and taking time to provide you suggestions and comments. Very much appreciated. Fred

Questions to ask your Supplier about Reliability

MTBF

Questions to ask a Supplier

Especially if they list MTBF on their data sheets.

My first questions, which I generally keep to myself, is ‘MTBF, yeah, right. Do they know better or not?” This is generally not a good way to start a conversation with a vendor about the reliability information you need to make appropriate decisions. Continue reading Questions to ask your Supplier about Reliability →

Why do we talk about reliability?

MTBF,

Why do we talk about reliability?

  • To make decisions
  • To estimate reliability
  • To understand risk

We talk about reliability because it matters. The ability to estimate reliability allows us to make design and development decisions. The ability to monitor reliability allows us to adjust the design, suppliers or expectations about a product. Continue reading Why do we talk about reliability? →

If not MTBF

MTBF

If not MTBF, then what should we use instead?

 

MTBF has issues. It is commonly mis-understood and mis-used. I find it hard to interpret and use for any meaningful discussion of reliability.

The entire premise of the NoMTBF site is to encourage you to not use MTBF.

There are exhaustive writings on setting meaningful goals and metrics in the business literature. A couple of tenants seem common: Continue reading If not MTBF →

Searching for MTBF

MTBF

Are you searching for MTBF?

I would ask why would you do that, yet I probably know.

You are looking for reliability information about a component or system. You want to know something about the expected failure rate or durability. Will it last long enough to meet your design and customer requirements?

Or, you have heard of MTBF and want to understand the acronym and metric. Maybe how to calculate the value from test results or field data. Continue reading Searching for MTBF →

MTBF Requirement Reaction

MTBF

Let’s talk about an appropriate MTBF Requirement Reaction

Every now and then we receive a customer request concerning reliability. If asked most customers would prefer no failures, low-cost of maintenance or ownership, and trouble-free long-term performance. And, many also realize that failures do occur. Thus a series of discussions occur to find the economically viable solution for both parties. Part of this discussion may include a poorly worded reliability requirement.

How you respond can help to improve the discussion and accelerate the finding of the right solution.

Continue reading MTBF Requirement Reaction →

Failure Dates not Rates

MTBF

Ask for failure dates not failure rates.

Just because the vendor provide the data convenient for an MTBF calculations should you settle?

No.

You have some questions to ask and some better information to gather. You may have a decision to make and using the best possible data helps you and your team make the right decision more often.

Continue reading Failure Dates not Rates →

Talking about Reliability

Engineering, MTBF

How do you talk about reliability?

“The language we use matters.” Wayne Nelson

When we talk about our products or equipment, we may refer to the expected durability of the system.

  • How long it will work before failure?
  • How long before we have to make repairs?
  • Will it work when we need it to work?

Our customers and investors also want to know how long will it last.

Continue reading Talking about Reliability →

Expecting MTBF

MTBF

What to Expect from MTBF

What do we really want?

When using the term, MTBF, many believe they are talking about the reliability of a device or system. A high MTBF numbers means it is a reliable item.

What we really want is the device to work over some duration without failure (or with few failures). It should perform a function as expected in the desired environment.

Continue reading Expecting MTBF →

Is MTBF a beginners metric?

MTBF

MTBF as stepping stone to better reliability metrics

MTBF is taught in many textbooks and introductory reliability courses. There may be some mention of Weibull and other distributions. You may even learn about the four functions: – reliability – probability density function (PDF) – cumulative density function (CDF) – hazard function

Continue reading Is MTBF a beginners metric? →