The reason so many use MTBF is because so many use MTBF. ‘Our data sheet has to include MTBF since all the other data sheets have MTBF’. Which seems to be primary reason MTBF is so common. It’s because it is so common.
Against this logic is the desire I have to use a measure of reliability that actually is understood. Using reliability (probability of success over a specified duration) as a measure seems some how odd or novel. It is easy to understand and it doesn’t obscure the reliability.
Doing what everyone else is doing and what customers are asking for is compelling. Using MTBF is easy, it is only one number, and with a few assumptions we don’t have to make any duration claims. The idea that we have to use MTBF since customers are requesting it is valid. Equally valid is to provide enough information that customers can truly understand any reliability claims or expectatIons.
When someone asks for MTBF, they are often asking for how long will a product last (a duration question) coupled with the assumption that there is a relatively low chance of failure over that time period. Saying something is ‘reliable’ generally means there is a low chance of failure over some period of time. By stating the probability of success and the duration, say 95% chance of surviving 5 years, is a very direct way to answer the query.
Instead, saying 97.5 years MTBF to answer the same question. 0.95 = exp [ – 5 / 97.5 ] Of course, 97.5 years sounds like a long time and it conveys that product is very reliable. Although it means that there is a 95% chance of surviving 5 years.
So, instead of stating 97.5 years MTBF for a five year product, instead say the product is 95% reliable at 5 years (or there is a 95% chance of the product surviving for 5 years). Better would be to include the entire life distribution and likely failure mechanisms. Yet, no one does that and we certainly don’t want to be unlike everyone else.
How do you request or specify reliability information? If you’re using MTBF, why?