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Example: Particle board factory. A control plan has been designed to asses complete lots of raw materials as they are received in the factory. Each batch has 30 items. As we want to control the individual values, the sample size is n = 1, and there are k = 30 groups. For this type of task, an Individual and Moving-range Chart (I/MR) is appropriate. In this case, the process is statistically under control: There are no individual points beyond the control limits or other suspicious patterns.
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In the production line, the humidity of raw materials is measured again before starting the manufacture of the board. Five samples of each lot are obtained, and the total number of lots is 20. For this task, an X¯ – R (x bar) Chart is appropriate: There is a point outside the control limits corresponding to group 16. The special cause must now be identified.
