How We Think About Reliability Is Important
Getting on an airplane we think about the very low probability of failure during the flight duration. This is how we think about reliability.
When buying a car we think about if the vehicle will leave us stranded along a deserted stretch of highway. When buy light bulbs for the hard to reach fixtures we consider paying a bit more to avoid having to drag out the ladder as often.
When we consider reliability as a customer does, we think about the possibility of failure over some duration.
And, we really don’t like it when something fails sooner than expected (or upon installation).
We Think About Reliability as a Probability and Duration
When someone advertises a low failure rate (or high reliability), we ask for how long. When we see an item has a 2,000 hour lifetime, it implies it also has a low failure rate (high reliability) over 2,000 hours of use.
When both the probability and duration are not explicit we fill in the blanks with our expectations.
If we, as consumers, think about reliability as a couplet of probability and duration, then shouldn’t we as designers, manufacturers, or producers do the same?
We should provide internal specifications that include both probability and duration. BTW: MTBF as in inverse failure rate is really a form of probability and has nothing to do with duration.
We should provide external reliability claims that are specific and include both probabilities and durations. Of course, if the function and environment are not clear, we should include that as well.
If we think about reliability in terms of the chance of failure over an expected duration, let’s make the conversation clear on those points. MTBF is not up to the task.
Improving the Reliability Conversation
We have the ability to ask for reliability information that is useful, meaningful, and aligned with the way we think about reliability. Ask of the probability of failure and the corresponding duration. Ask for various probabilities at different durations. Do not accept MTBF as it is insufficient when compared to how we think about reliability.
If someone offers MTBF as reliability, ask over what duration it is valid. (Also ask why they would want to use MTBF, yet that is a topic of another post.)
If someone offers an item will last for 2 years, or that the warranty is 2 years… that doesn’t mean there is a low probability of failure over two years, it’s just a duration. Ask for the probability element to get a better understanding of the offered reliability. Warranty durations just mean they will (or are supposed to) fix or replace an item, not how often they expect to have to do so.
If I had enough trust and understanding that an item would last with a very low chance of failure for 2 years, I wouldn’t need the warranty. If unsure, I’d expect a full warranty and it should cover the entire period of time I expect to use the item.
As a producer provide better information concerning reliability. Warranty terms do not describe reliability. A duration or vague claims of ‘high reliability’ are likewise insufficient.
Provide both the duration and associated probability of successful operation over the entire duration. Better is to provide various couplets of probability and duration. Even better is provide the expected life distribution including the environment and use profile parameters.
If someone asks you for reliability information, they are thinking of probability and duration. Give an answer that provides probability and duration. It is the way we think about reliability.
If someone is asking for availability or maintainability, that is not a probability of successful operation over some duration. Yet, providing just MTBF doesn’t work either. Provide suitable information so your customers can compare your product’s expected reliability performance with what they think it should be. Help your customers make informed decisions concerning reliability.
Extend this improve communication around reliability to your vendors – ask for and provide complete information concerning reliability. Again, MTBF is not sufficient, nor useful. If you want to improve the reliability performance of your product you need to get reliable parts from your vendors. This does not just mean higher MTBF values. It means a lower probability of failure over the duration of desired operation.
Talk About Reliability as People Think About Reliability
You can improve the communication around reliability by insisting on using probability and duration every time you talk about reliability. Set the standard. Set an example.
Doing so you will find others embrace reliability discussions as it make sense. If we discuss reliability as we already are thinking about it, good thinks happen.
What’s your observation here? How do you find yourself thinking about reliability? Add you comments below.
A small help, when we don’t have proper bill of material how to excute the reliability analysis. Please clarify
Fred Schenkelberg says
Hi Viswanath, you can and should start with reliability objectives and models at the system level (often based on functional elements or subsystems) Also do a system level FMEA to identify system level risks to reliability performance. There is plenty and can and should being doing prior to having a detailed BOM – and the impact on final reliability is much larger the earlier in the program you engage. Cheers, Fred