The following note and question appear in my email the other day. I had given the definition of reliability quite a bit of thought, yet have not really thought too much about a definition of ‘product life time’.
So after answering Najib’s question I thought it may make a good conversation starter here. Give it a quite read, and add how you would answer the questions Najib poses.
Many thank for your very interesting articles concerning reliability topics.
I was wondering if there was a common definition of the product life time.
I know that there are several standard which specify a definition of the MTBF.
What about the product life time?
Do you know a standard which defines the product life time?
I look forward to hearing your feedback.
Thanks for the note and question. I do not have a clear definition of product life time, and maybe should write one.
MTBF, as you know, is not product lifetime, it is a measure of the inverse of the chance of failure per unit time.
Customers define or set expectations concerning product lifetime. If I expect my desktop computer to last 5 years, that is life time that I want and expect. Product/system development teams have to understand what the customer wants/expects in order to deliver a product that lasts long enough to meet the customer’s expectation.
Here are some related definitions from my upcoming glossary on reliability terms.
Asset life: The period of time from asset creation to end of life or retirement.
End of life: The instant an item fails to perform its intended function within desired specifications with no recourse for corrective maintenance. May also refer to the retirement or decommissioning of an item even if still in working order.
Field lifetime: The expected duration an item is expected to function or provide value for the customer.
Life: See conditional probability of failure, or probability of survival. Often stated as a duration missing the probability element. Together duration and probability define a statement of life.
Useful life: The number of life units from manufacture to when the item has an unrepairable failure or unacceptable failure rate.
Generally, the term product lifetime is from the customer or end user perspective, and is often defined as the time between installation, commissioning or startup, till removed, decommissioned, or failure.
In other words, how long will the item work as expected for the customer?
hope that helps,
Often when I hear someone talking about a product’s life or the life of a system I think about duration. The part that is missing is the probability of successful operation over that duration. To say something has a 5 year lifetime really is pretty meaningless unless you also state the chance an item will survive that same 5 years.
For repairable systems, such as my car, when one says it will last 10 years, that implied that includes the regular maintenance and maybe a trip to the body shop or a new windshield or two. We are often more interested in availability and cost of ownership, not just reliability. Again the life time statement on its own is not all that helpful.
As you know, how we define the start and end of product life is often rather fuzzy. Is it on first power up in the factory or by the customer? Is the end of life when shut down or disconnected, or when sent out with the trash? Many times we do not know when a product ends its period of use by a customer unless we take deliberate actions to gather that information.
Product life time in some circles bounds the duration from concept to the retirement of the last product. Others define it as the warranty period or the duration we produce and support a product.
I do not have a single definition that all of us may agree upon. Still looking for that better answer for Najib. Do you have a better answer?
Use the comment section below to provide your answer, comments, or additional questions concerning a definition of product life time.