Question

In quality control, statistical process control (SPC ) is often employed to analyze the process capability in manufacturing or service operations. Explain the following with an example each :

• What is a p chart  ?
• What is a c chart ?

There are 3 machines in your factory which can produce a ruler. The rulers as produced has a spread in length between 12.5 thru 12.4 inch.

The standard deviation of the 3 machines are

* 0.15

* 0.09

* 0.13

Please calculate the respective process capabilities

Given the standard deviation of the 3 machines are as follows, calculate the process capability of each machine. Which of the 3 machines has the best process capability index ?

Answer 1

Answer-

A p chart is a control chart for measuring process variability when the data being measured is attribute data. An attribute data alludes to discrete quantity values such as count, proportion, number, etc. In this case a p chart usually uses samples and the count of defect. The count of defect separated by sample size for each observation is considered as fraction and the p (stands for proportion) is calculated.

These proportion values are then used to calculate as far as possible and plotted p values are measured against them. For example, a machine produces 5 defects in the product when 1000 units are seen in sample 1 then it produces 3 defects in the following sample of 500 observation. The p values are 5/1000 = 0.005 and 0.006 respectively.

A c chart is another control chart for measuring attribute data. Here, c stands for count. In contrast to the previous approach of p chart, c chart directly counts the quantity of defects for each sample.

These counts are then used to calculate control cutoff points and then the c values are watched. For example, a plant performs quality control by taking samples of 500 units consistently.

The defects saw in the initial 2 hours are 2 and 6. The c values for 2 and 6 for the initial two samples.

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