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 GetCalculatedValueRecord(1) returning wrong value

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blazermaniac Posted - 21 Jul 2016 : 18:52:26
Given the primary points 1.155 and 1.141 a call to (double)this.ChartData.GetCalculatedValueRecord(1).CalculatedValues.GetValue(CalculatedValueIndex) returns 0.014000000000000012 while I expected 0.014. The control limit is 0.014 which results in a violation. I found that this is an artifact of using doubles (base 2). I have cast the arguments in SetCalculatedValue, case SPC_INDIVIDUAL_ABS_RANGE_CALC as decimal (base 10) to fix the problem.

If you have a better solution please share it with me.

Thanks!
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blazermaniac Posted - 22 Jul 2016 : 12:06:07
I understand your point but on the chart we are reporting a false negative and when you look at the alarm notes it appears the limit and value are equal. For our implementation we compare the values and if OOC we perform some various actions which include at a minimum sending an email alert and up to putting a lot/tool on hold.

In our case at least the code change is necessary.

Thanks,
Brian
quinncurtis Posted - 22 Jul 2016 : 08:59:25
Statistically, as far as the process goes, I don't think your change will have any affect on the overall control of the process. SPC Control Limits are not perfection. Control limit violations are only strong (not infallible) hints that the process is out of control. There will always be false positives and false negatives. If your measured variable tests within 0.0000000000000001 percent (the floating point accuracy of a double) of a control limit, then assuming that the process is out of control is a far better assumption to make than that it is in control. That's the way a Quality Control Engineer would look at it, as opposed to a programmer. Look at it another way - if you are a doctor, and you were taught in school that a fever of 106 degrees F will most likely kill your patient, but the patient only has a fever of 105.99999999999999999 F, do you ignore the fever, or treat it as if it were life threatening? Just my opinion though.

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