P_Control_Chart.xls

I have created a "P Control Chart" in Excel Spreadsheet for different Sample Size. But unable to get the accurate graph as Sample Size varies, the control limits change. In case of "Zero" defects from "x" sample size the process should be "Under Control" whereas the graph depicts "Out of Control" because the LCL is "0.08" but Process "p" is "0.00" (Please see row 8 in attached spread sheet).

Likewise, there are couple of other samples where it shows Process is out of control as I am unable to make LCL zero when sample size is varying each time.

Please suggest me the way out or facilitate me an excel spread sheet which provides accurate chart when Sample Size Varies each time.

Dear Santhosh,

Please note that your LCL may not be necessarily Zero. If it is not zero, it means that if you get defects less than LCL, it indicates one of the two things.

• Unreliable data.
  
• If data is ok, it indicates a special (good) cause. You may like to investigate and learn what has improved. I think you should be concerned about reasons of all surprise elements (pleasant or unpleasant).
  

Regards,

VK

Hi,

I need your assistance on judging which Control Chart to consider for Variable Sample Size and Defectives, as I referred few websites which lead me to more confusions since different websites illustrated different Control Charts consideration.

Please suggest.

 

Santhosh,

The correct control chart for variable sample size and defectives is p chart. Crossing a lower limit in a defects or defectives control chart is not a worrisome issue. (as you are charting defects/defectives - lower the better). However, in such a case, you may like to investigate the reason for an extraordinarily good performance.

Regards,

VK

 

If you want to calculate control limits for the p chart, you can use this calculator - https://www.benchmarksixsigma.com/calculators/control-limits-for-p-chart/