Variability Basics - Process Time Variability
5 important questions on Variability Basics - Process Time Variability
Why is variance or standard deviation in itself not a good measure of variability?
Because they are absolute measures of variability and do not say anything about the relevance, or 'severity', of the variation.
Are the following situations or things described by low (LV), moderate (MV) or high variability (HV)?
- A: An SCV of 1,4
- B: A process with
and
- C: All repair times in a certain machine's 'lifetime' of respectively 20, 29, 5 & 11 minutes
- D: A poisson arrival process with a mean of 2
- E: The interarrival times of the above process described at D
- F: Process times with very long outages
Use table 8.1 for clarification on LV/MV/HV.
A: MV (CV = SCV^0,5 = 1,4^0,5 = 1,18)
B: LV (CV = 1,2 / 2 = 0,6)
C: LV (standard deviation = 9,09, mean = 16,25, CV = 9,09 / 16,25 = 0,56)
D: LV (std. deviation in poisson is equal to the square root of the mean, so CV = 2^0,5 / 2 = 0,7)
E: MV (interarrival times of a poisson arrival process are exponentially distributed -> CV = 1)
F: HV (in general, see table 8.1)
Is a CV of 0,75 LV or MV? Does it matter if you had this question wrong or right?
It is MV (see table 8.1).
It does not really matter, what counts is the value of the CV and its effects on the process, not the 'label' you give it.
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What is the CV of a deterministic process?
CV = 0. A deterministic process is a process where there is only one possible output. Hence, there is no variability.
There is a confusing mistake in 8.3.1. on page 268, which?
On the last line of the page it says: " denotes the variance", but it should really be "
denotes the standard deviation".
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