A note from Scott – providing feedback on the NoMTBF site.
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.
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.
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.
Vice President of Quality and Customer Excellence
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