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Prasoon Bhargav

P Chart

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Dear All,

Please see below a query posted by a past participant. Please feel free to respond to this query.

Regards,

shantanu kumar

--------------x--------------------------x-----------------------x--------------------------x--------------------------x----

Hi Shantanu,

Hope you are doing well.

I was trying to understand p-charts. Consider a case where in the output is varying and defectives are identified. Data for the same is as follows. When I tried to plot a p-chart for the same in Minitab, the UCL line varies as shown because of varying output, but a value of UCL (0.05439) is highlighted; Similarly for LCL (0.00675). How is this figure arrived at?

Defective Volume

15 500

20 528

13 465

12 470

Warm Regards,

Krishnan S

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The UCL and the LCL shown are for the last data point (i,e, Defective = 12 and Volume = 470).

 

How do we arrive at these figures is as follows

 

1. First calculate pbar => find out individual proportions and then take the average of the same.

2. Control limits = pbar +- 3 times (square root(pbar(1-pbar)/N), N = sample size for the last sample (if you change the last data point, you UCL and LCL would change).

 

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