CONTROL CHARTS

Control charts are statistical tools under, Statstical Process Control System, to control whether a process and its varibles are under statistical control or not therby ensuring consistency in product or service quality.

In production process it is required to monitor the extent to which our product meet specification.

there are two reasons for the inferior quality of product are
1) Deviations from target specification.
2) Excessive variablity around the target specifications.

usually varitations in process is caused by

Common Causes :- Variations caused due to the system, they can not be reduced unless the process itself is changed.
It influence in the same way, they produce random or natural variation in the data pattern, when it is free from special causes.

Variations due to common errors are result of the combined effect, rendering it difficult to identify and eliminate without further investement and commitment of managment for improvement.

Few examples of common causes:-

• The KSA of employees are below the requirement
• Absence of suitable work environment and work culture.
• Poor methods or implementation of methods.
• Ambiguous/ unclear instructions and delegations.
• Ill maintained mahcinery, tools and equipments.
• Lack of machine capability.

Special Causes :- variations caused by some special factor which is not because of system, spececial causes shows unusual pattern of variations and occur intermittenly, special causes can be found and rectified without changing the system. It is easier to eliminate special cause and management role is even not required.

Few examples of special causes

Operator not using right setting of machine.
Irregular or careless employee.
Assembly of machines not being done properly.
A tool, machine part etc is not working right.

for making a control charts we simply extract samples of a certain size then produce line charts of the variablity in those samples and consider their closeness to the target specifications.

If a trend emerges in those lines or if samples falls outside prespecified limits then process is considered to be out of control and take action to find the cause of the problem

STEPS OF MAKING CONTROL CHARTS

1 analyzing the sample and the problem at hand, identify the nature of the data i.e whether the data represent variable data or attribte data, in case the data is variable then it represents the defects or defective.

2 once you had identified the nature of the data you have to find out which chart will be made, which is decided as per the following manner

In case it is Variable Data then

if size of sample, (here represented as n) is greater than 10
then make X bar S type chart

if size of sample, n is less than 10
then make X bar R type chart

If size of sample, n is equal to 1,
then make X-R chart.

In case we have attribute data and it represent defectives then we will make following chart, in this case we will have multiple samples

if the size of sample n is constant for all samples taken then
p type or np type chart will be made

in case sample size n is variable then
p type chart will be made

In case the data is for defects then charts will be made according to following

in case the sample size n, for all the sample is constant then
c type or u type charts will be made

in case the sample size n is variable then
u type of chart will be made.