Statistical quality control (SQC)is defined as:
“The monitoring of quality by application of statistical method in all stages of production.”
Statistical methods of investigation are based on the theory of probability.
It relates to the characteristic of product from both qualitative and quantitative point of views to meet the established standards.
It has been used to serve:
• As a basis for improved evaluation of materials through more representative sampling technique
• As a means of achieving sharper control in certain manufacturing processes
• To provide logical approach to variations
Selection:Selection of appropriate method depends on:
• Type of measurement
• Sampling techniques
• Design of Experiments
• Type of Sample distribution
The procedure consists of:
• Proper sampling of product
• Determining quality variations of the sample
• Making inferences to the entire batch under investigation from the observed data
• Once the characteristic data pattern of a process has been determined, the pattern can be utilized to predict the limits within which future data can be expected to fall as a matter of chance, and to determine when significant variations in the process have taken place.
Data can be analyzed by using appropriate method of analysis:
t-test for comparison of two populations. T-value is calculated and from t-value the P-value is noted from the table:
P>0.05; test is non-significant
P<0.05; test is significant.
It means analysis of variance and is used for comparison of more than 2 parameters.
The objective is to determine whether the major source of observed variations is by chance or assignable.
Types of variations:
These variations are inevitable because any program of production and inspection has its own unique chance causes of variations which can not be controlled or eliminated and often cannot be identified.
These variations can usually be detected and corrected by statistical techniques. Assignable variations are usually caused by machine or a specific batch of production or a container.
Thus the use of SQC permits the:
• Evaluation of magnitude of chance variation of product quality.
• Detection of assignable variations of product quality by means of QC charts.