Calculating $\bar c $ in C chart In studying C chart I came accross this problem in Statistical Process control by Douglas .C.Montgomary.   
In this exercise it says:
A control chart is used to control the fraction of non confirming.Ten subgroups yield the following data.Construct a control chart for the number of non confirming in samples of n=100 

here we can construct a C chart.My question is here $\bar c$ is calculated as $n\bar p$.  
But in another example presented in the book  
$\bar c$ is calculated as total no.of defectives/no.of samples
In the first exercise if I use total no.of defectives/no.of samples I get  a wrong answer.  
Can someone please explain to me why two different ways were used in these two to calculate $\bar c$ and how to determine which one of these should be used
 A: The purpose of a $c$ chart is to count the nonconformities per unit.  The chart is designed for when the unit size is constant.  Its companion is the $u$ chart, which is also used to count the nonconformances, but only when the unit size changes.
I could only speculate as to why the error is there and why it has not been fixed.  I have a book on control charts where they calculate the wrong value for the sample problem but have the value correct in the table.  I had a Statics and Strengths of materials book where only about 50% of the answers in the back of the book were correct—and it was the third edition of the book.
The best way to check on something like this is to check a couple of sources, including your text (read what the chart is supposed to plot, beyond what is written in the example problem).  These charts and how to calculate their values are widely published online and in print.
Now that you know about the error, write the publisher and author—they can tell you better than anyone else why the error is there.
