Principle (Process Times): Defect rate is increasing in the means and standard deviation of the individual process times at all stations.
Motivation
As noted before whenever a station in the line fails to complete its task within cycle time, a defect occurs. The principle states that as the mean or standard deviation of the process time in any of the station in the line increases, the defect rate increases. This is because increasing either of these attributes increases the likelihood of failing to complete the task during the cycle time.
Example
To see how this happens we look at a line with a cycle time of 20 seconds
and the following two distributions of the processing time at a station.
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The processing time in the first station, shown in blue in the above figure, follows a Normal distribution with mean = 10 seconds and standard deviation = 2.45 seconds. The second station has a Weibull distribution with mean = 10 seconds and standard deviation = 5 seconds, shown in red in the figure. The cycle time of the line is set at 20 seconds. The probability of producing a defective part is can be seen graphically as the area below the probability distribution curve beyond the cycle time of the line. As is obvious from the figure, the second station has a much higher probability of producing a defective item. This illustrates how increasing the standard deviation increases the defect rate. Obviously, the second station would be even worse (more likely to fail to complete an item during the cycle time) if its mean were larger than 10 seconds.