Best Fit Product - Emotional Engineering - page 25

Fig. 1 Bathtub curve (from Ref. [ 1 ])

operation to operation, so it was relatively easy to keep products operating in good
conditions.
Why does the above sentence use plurals; products, not a product? It is because
yesterday products were produced in mass and we were interested in how a group
of products with identical or similar functions work. We were not particularly
interested in an individual product.
This will be well understood if we consider Reliability Engineering of yesterday
about failure rates. Reliability Engineering provides us the famous guideline called
bathtub curve [ 1 ] shown in Fig. 1 .
At the initial stage of operation the failure rate is high but it will decrease very
quickly. After this decreasing failure rate period (DFR), the failure rate stabilizes,
which is known as constant failure rate period (CFR). Then increasing failure rate
(IFR) period follows.
Since failure rates are high at the initial stage just after delivery, how we can
shorten the decreasing failure rate period and enjoy the stable operating conditions
longer was a very important issue these days. Burn-in is an activity to exercise
products prior to putting them into operation in order to remove early failures.
It should be remembered that the Bathtub Curve is a graph showing failure rate
changes of a group of products. CFR means the ratio of the number of failing
machines against the number of working machines remain the same as can be
understood from the following de ! nition of failure rate.

Failure Rate ¼ Number of Failing Products ð Þ =

ðNumber of Remaining Working ProductsÞ

ð 2 Þ

Thus, it does not mean your own product will work very well in operation because
Bathtub Curve only provides a guideline for operations of a group of products.