These factors are very frequent and can be statistically evaluated.
These are determined by external variations, their causes are out of statistics control. Even though these factors are out of statistics control various test are used to determine their occurrence. Their occurrence is not frequent (Wise, 1998)
2) Tabulation of collected data; the data about the activity and the time is carefully tabulated. It is easy to read tabulated data in addition to the fact that the data can be grouped simplifying statistics calculations.
3) Calculation of the mean; this aids in determining the time to allocate to each activity; different activities will have different means. The time allocated for each activity can be based on the mean.
To define the control limits it is necessary to evaluate the history of the process and also determine how wide the control limits will be set. To determine the limits, it is common to draw the normal distribution curve and attempt to normalize the curve. Probability density functions are used.
Wise, S. A. (1998). Innovative Control Charting: Practical SPC Solutions for Today's Manufacturing Environment. Milwaukee: ASQ Quality