Statistical Quality Control
Introduction
- Growing competition
- Watch over quality
- Mass production unable to check the quality at each and every level
- SQC refers to use of statistical techniques in controlling the quality of manufacturing goods
Definition
- SQC should be viewed as a kit of tools which may influence decision to the function of specification , production or inspection
- An economic and effective system of maintaining and improving the quality of outputs throughout the whole operating process of specif. , prod. & insp. Based on continuous testing with random samples
Characteristic
- Control quality standard
- Exercise during production process
- Determine the variation
- Statistical charts
Advantages
- Quality under control or not
- Eliminate assignable causes
- Lower inspection cost
- Protect interest of consumer
- Healthy influence on the psychology of workers
- Earn the goodwill
- Guide for new plant
Causes & Methods
Causes of variation
- Assignable causes
- Chance causes
Methods of SQC
- Process control
- Product control
Acceptance Sampling
Acceptance sampling is concerned with the decision to accept a mass of non – conforming the quality. The decision is reached through sampling
Risks
How to conduct Acceptance sampling or Types of sampling Inspection Plan
- Single Sampling Plan
- Double Sampling Plan
- Multiple Sampling Plan
Operating Characteristic Curve (OC Curve)
- Graphic measure of assessing the ability of sampling plan in distinguishing between good and bad items
- Depicts the relationship between the probability of acceptance the lot for different lot quality expressed in terms of percentage defectives
Control Charts
- +3Sigma Upper Control Limit
- Central Control Limit
- -3Sigma Lower Control Limit
Purpose & uses of control charts
- Determining the quality standards
- Detecting assignable & Chance Variation
- Reveals variation from desired level
- Necessary steps for correction
- simple to construct & easy to interpret
- Less inspection cost
Types of control charts
- On the basis of Variables
- Mean Chart
- Range Chart
- SD Chart
- On the Basis of Attributes
- p Chart (Fraction Chart)
- np Chart (No of defects)
- C Chart (No of defects per unit)
