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What is it: A C-Chart is used for monitoring and tracking the number of times a defect occurs, relative to a constant sample size, a given sample can have more than one instance of the condition, in which case we count all the times it occurs in the sample.

Why use it: To maintain high quality standards, companies need to have a clear way to see the number of defects resulting from a process.  Whether the process is in shoemaking, waiting tables, or molding aluminum, something can happen within the process to make the product fall outside specifications. How do we communicate defects and show if there has been a change in the performance of a process?  We can develop a count chart, or c chart.  A C-Chart graphs the number of defects in a product or service.  Once the C-Chart is developed, management can use the information to determine whether a process is in control. A C-Chart shows the number of defects in each sample, or inspection unit, of uniform products. 

When to use it: This information allows a manager or employee to identify nonrandom patterns, and to further evaluate whether a process is no longer in control.

How to use it: The data will be graphed once it is collected to show a clear, visual representation of the number of defects. Data may be collected in one of two ways. First, we may collect data regarding the number of defects per unit, and second, we may track the number of defects per inspection lot. The data is then graphed in a spreadsheet, with each point connected by a line. The x-axis represents the number of inspected products, or the individual unit number. The y-axis shows the number of defects or the number that failed inspection. Once the data has been collected and graphed, upper and lower control limits should be determined, and the average number of defects calculated. 

Find the average number of defects per unit, or per lot. This is c bar. Next, add a line on the graph representing c bar. This is the central line. The next step is to determine upper and lower limits. First, find the standard deviation by calculating the square root of c bar. Then, add c bar to three times the standard deviation. To find the lower control limit, instead of adding three times the standard deviation, subtract it.  If the lower limit is negative, then there is no lower limit. Add the lower and upper control limit lines to the graph. If the C-Chart shows that the number of defects is outside the upper or lower limits, or that the process is producing defects in a nonrandom pattern, the process should be stopped until it is evaluated and adjusted to prevent future defects.

Preview (Click to enlarge)
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Microsoft Excel Format
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Adobe PDF Format
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c Chart Production Template
USD $14.95
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