The other day I was thinking about my Reliability Blog and it led to my thinking about CURVES, especially those most common in Reliability Engineering. We regularly use the Gaussian function and the Weibull Chart, but as far as my experience goes, the Bathtub Curve has been the most popular way to visually summarize the lifetime expectations of just about everything. Then, looking at the curve I could not avoid noting that it uses words most common in personal daily life: Infant Mortality, Useful Life and End of Life, and that stimulated thought on life at the beginning, middle and end.
Then Rob Reiner’s 1989 Movie, “When Harry met Sally” popped into my head, not only as a way of getting your attention, but because anyone who saw the movie can undoubtedly recall “that” scene where Sally (Meg Ryan) challenges Harry (Billy Crystal) that women can deceive men by faking an orgasm and so saying Sally “fakes” a very public (and very persuasive) orgasm to convince Harry. After Ms. Ryan is through with her demonstration, a nearby customer, (Estelle Reiner, Rob Reiner’s mother), when asked by a waiter what she wanted, replies, “I’ll have what she’s having” (33rd on the list led by “Frankly my dear, I don’t give a damn”). Now, how does this relate to the Bathtub Curve? Well, if Harry and Sally link up and Harry impregnates Sally, do we then have the beginning of a Bathtub Curve? Probably not. Bathtub Curves are by definition the lifetime of a population of products (or people, et al) using a graphical representation. What do you think?
Rather than take up a lot more of your time on this issue I would like to refer you to some very well written material on Bathtub Curves and then have you comment on how you see Bathtub Curves and what purpose they have played for you in the past, the present and how you see the future.
My first source was Wikipedia: http://en.wikipedia.org/wiki/Bathtub_Curve
My second and I believe the best written and probably the most informative source is a 2-part paper written by Dennis Wilkins while he was at Hewlett-Packard (reportedly now a consultant with ReliaSoft):