THE 7 QC TOOLS

Control Charts

Variability is inherent in all manufacturing processes. These variations may be due to two causes:

i. Random / Chance causes (un-preventable).

ii.Assignable causes (preventable).

Control charts was developed by Dr. Walter A. Shewhart during 1920's while he was with Bell Telephone Laboratories.

These charts separate out assignable causes.

Control chart makes possible the diagnosis and correction of many production troubles and brings substantial improvements in the quality of the products and reduction of spoilage and rework.

It tells us when to leave a process alone as well as when to take action to correct trouble.

BASIC CONCEPTS

a. Data is of two types :

Variable - measured and expressed quantitatively

Attribute - quantitative

b. Mean and Range :

X - Mean is the average of a sub-group

R - Range is the difference between the minimum and maximum in a sub-group

c. Control Charts for Variables

Charts depleting the variations in X and R with time are known as X and R charts. X and R charts are used for variable data when the sample size of the subgroup is 2-5. When the subgroup size is larger, s Charts are used instead of R charts where s is the standard deviation of the subgroup.

d. Control Charts for Attributes

The control charts for attributes are p-chart, np-chart, c-chart and u-chart. Control charts for defectives are p and np charts. P charts are used when the sample size is constant and np charts are used when the sample size is variable. In the case where the number of defects is the data available for plotting, c and u charts are used. If the sample size is constant, c charts are used and u charts are used for variable sample sizes.