There is a lot more to "has it been replaced"
If the same question was asked of me, with regards to my HR10-250... I had it replaced 3 times, before I gave up and found my own solution. But that was nearly 2 years ago.
As for the HR20...
When did you have it replaced? And Why? And By who's recommendation?
All factor into it... the "value" of that 18% number.
You are so right. If boxes repalced were truely the problem, the odds of the second one being defective as well would be: 18.6% x 18.6% or 3.4%. It is just like the odds of rolling snake eyes (two ones) is 1/6 x 1/6.
So if it really were the hardware in the first place with a failure rate of 18.6%, the failure rate on the replacement units should be 3.4%. It isn't. The failure rate on the second units is 27%.
That shows that for most of the replacements hardware was not the cause. At a previous company, the service manager said that the most important reason for sending out a replacement is to force the user to re-attach the cables and hopefully snug them up this time.
Here are two of the statements from the analysis:
"The replacement rate on the HR20 is high. But notice that the replacements often do not help. This would indicate that the problems are often not due to the hardware."
"While this sampling is large, it does not represent a random sampling of all HR20 users."
This group is far from average.