# Charts For Attributes Such As (P,c Charts)

**The ***p ***chart:**

For each sample (subgroup) the failure proportion *(p) *is calculated and charted in the control chart. The failure proportion is calculated as shown below:

Where:

*NF*=number of failures in the sample

*n*=sample size (number inspected in sub group)

Construction of control limits is done as follows:

Where:

And:

*TNF*=Total Number of Failures in all the samples inspected

*TNI*=Total Number Inspected (the sum of all samples).

- For varying sample sizes the control limits vary from sample to sample.
- If varying control limits may give problems to the users then plan for fixed sample sizes.
- For small variations (±20%) using the average sample size is recommended. The benefit of using the average sample size is that the control limits are constant from sample to sample.

**The np chart:**

For each sample the number of failures (*np*=the number of non-conforming units) is counted and charted in the control chart. Construction of the control limits is done as follows: