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Control Charts-7 QC Tools

Control Charts-7 QC Tools

Control Charts used for M Chart & R Chart, P & C-Chart and X-Bar Charts.

What is a control chart?

Control chart signifies the provision when data goes beyond control limit and inspires the process owner to identify the assignable causes of the variation in the process to take immediate corrective action to control the process.

Why does control chart use as a QC tools?

  • A defect as a result of process variation and variation comes in the process because of assignable and common causes.
  • When data goes beyond control limits. It is because of assignable or special causes and it requires immediate corrective action to maintain the process within control limits.
  • Hence to maintain the process variation within control limits process owner requires a control chart.
           X-Bar Table:
X-Bar Table for X-Bar Chart or Control chart

       X-Bar Chart:
X-Bar Chart used in control chart

        Types of control chart.

There are two types of control charts.
1) Control charts for variables such as length weight volume etc. It is further subdivided into two categories.
  • Main chart for accuracy whereas accuracy is the closeness of average value to the target value.
  • Range chart for precision (Precision is the closeness of individual values.)
       R-Bar Table:
R-Bar Table used for control chart
        R-Bar Chart:
R-Bar Chart used for control chart
       Control charts for attributes such as (go, no-go), (OK, not OK), (good, bad) etc. It is   further subdivided into two types.
  •        P chart for proportion defective.
  •        C chart for defect count 
       P charts and C charts are used for attribute inspection in terms of is there a defect is there not a   defect
        P chart:
       P chart is used to monitor the percent or fraction of defective units within a sample.
       When you're thinking about P charts think percent defective we have a total number of items in a   sample and we're just counting how many of them are defective when we go to create the upper and   lower control limits just like we've done before we're going to take the mean and we're going to add   a number of standard deviations the mean and then we're going to take the mean and subtract the   number of standard deviations the mean for the lower control limit one thing to remember with a P   chart is we actually have to calculate the standard deviation of P and to do that we need the mean   percent effective or the average percent effective and we also need the mean sample size so in this   case it would be the square root of P bar times 1 minus P bar over N bar now typically we use 3 for Z because we typically use three standard deviations away from the mean.
UCLp = ‾p + Zσp
LCLp = ‾p – Zσp

Where ‾p= Mean percent defective.
            ‾n= Mean sample size.
Typically Z=3(3 std. Deviation from the mean)

C chart
 C chart is used to monitor the actual number (count) of defects within a sample there can be more than one defects on a unit within the sample.

Where ‾c= Mean # defects

Typically Z=3(3 std. Deviation from the mean)


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