[chbot] Is there a statistician in the house?

Stephen Irons stephen at irons.nz
Mon Oct 19 01:18:18 BST 2020

We have just had a discussion about whether the test 'finds' or 
'causes' the failure, and have concluded that the statement was 
meaningless, because there was no specification about expected 

For example, we want to test that a device input works correctly at its 
maximum specified operating voltage. The test is to apply 220 V to the 
device input. After the test, the input no longer works. Is this a 
if the input was rated for 12 V, no it is not a failure - the test 
exceeded the design specification, and we should change the test.if the 
input was rated for 500 V, yes it is a failure - the test was within 
the specification, and we conclude that something is wrong with the 
unit under test; it might be a design issue or a manufacturing issue
Our response to the client was essentially that: we have found a test 
that makes devices fail in the same way that they are failing in the 
field. However
there was no design specification for that parameterwe don't know 
whether the failing devices are being used within the design 
specification for that parameterwe don't know whether our test is 
within the design specification for that parameteras a result, we don't 
know whether the problem is a test issue, a design issue or a 
manufacturing issue
For interest, the parameter is 'shock and vibration'. The test is to 
load a number of known-good units into a vibration chamber 
(essentially, a tumble-drier with the heater turned off), run it for a 
few hours, and see how many still work after that.

The result: 70% failed after 4 hours, and the devices are in a similar 
failed state to failures in the field.

But we have no idea what level of shock and vibration the units are 
experiencing in the field, or whether our test is in the same ballpark 
as that, or whether either of these are close to the (unknown) design 
limits, or whether the manufacturer is using a part that does not meet 
the design criteria.

Stephen Irons

On Sun, Oct 18, 2020 at 22:39, Marshland Engineering 
<marshland at marshland.co.nz> wrote:
>>> But we now have a repeatable test that causes the failure.
> I'd find a new test!!!
> I'm sure you mean 'But we now have a repeatable test that finds the 
> failure.'
> Cheers Wallace
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