Postmarket Surveillance - Medical Device Design: Innovation from Concept to Market - page 289

Random Growth

Upper limit

Lower limit

Sample

Figure 12.2
Example process control chart for a device.

Number
of
failures

“Bathtub”

Time to
failure

Figure 12.3
Typical bathtub curve of failures.

which not only led to normal bio-burden results but also a more consistent finish on all
components. 2

12.3.2 Reliability - Bathtub Curve
The Weibull function is over 60 years old - it was established in the 1950s. Ever since, it has
been used been to analyze and predict failures. However, there is a more simple process one
can use initially and this is the life bathtub plot. For simple components (especially those
solely electronic based) the life follows a specific shape - as illustrated in Figure 12.3 .

2 You would be amazed at the occurrence of this type of issue. All is fine because “George” has always made
things brilliantly, but on his retirement the new guy works from the existing drawings - which did not match
George's delivery…they had never needed to. The drawings or component specification must match the expected
outcome.