Control Chart in 7 QC Tools

 

What is Control Chart?

 - It is a graph used to study the process changes over time.

 - It is a statistical tool measures variation and show it to you graphically.

 - Control chart also called as Shewhart chart.

 - Control chart used to distinguish between process variation resulting from common causes and variation resulting from special causes.

Purpose of Control Chart

 - Control chart indicates whether a process is in control or out of control.

 - It determines process variability and detects unusual variation taking place in a process.

 - It provides information about the selection of process and setting of tolerance limits.

History of Control Chart

 - Control chart was invented by Walter A. Shewhart in the '1920s. 

 - To reduce the frequency of failures and repairs in telephony transmission system, the engineers find the up-down variation in the process by using control chart.

 - Action should be taken on the variation that have negative effect on business.

 - Shewhart framed the problem in terms of common cause and special cause.

Types of Control Charts

 - There are main 2 types of charts attribute and variable.

 - Variable charts are used for measurable quantities. Attribute charts are used for attribute data like number of defects per unit or number of defective items.

 - Variable chart contain XMR chart, Xbar R chart and Xbar S Chart.

 - Attribute chart contain np chart, p chart, c chart and u chart.

 - This classification depends on sub groups, recorded data is variable / attribute, defects (occurrence) and defectives (Pieces).

How to draw control chart?

1. Collect the data

 - Record the readings and classify it into subgroups as per sampling plan.

2. Calculate subgroup average and overall average

 - In this step we find the average of subgroup (X bar) and overall average i.e X double bar

3. Calculate Subgroup range and average of range

 - First we find the individual subgroup range and then average range of all subgroups.

4. Calculate lower and upper control limits

 - In this step we have to find the upper and lower control limits for X and R chart.

5. Plot the graph

 - Draw the X axis, Y axis and center line.

 - X axis shows subgroup number, Y axis shows X bar and R bar values. center line shows X double bar and R bar value.

 - Draw control limits, upper and lower side of center line.

 - Plot the points and join the point.

 - Give name to the chart.

Benefits of Control Chart

 - Simple graphical tool for process performance monitoring.

 - They are designed to identify which type of variation exists within a process.

 - Control chart highlight areas of performance that may require further investigation.

Click here:-  Control Chart Interpretation, Rules and Patterns