7. Statistical Quality Control

7. Statistical Quality Control

Objective :

• To know the basics of statistics.
• To understand different data types and analyze & interpret the data.

7.1 Statistical Quality Control :

Important points discussed in this Topic :

1. The concept of Variation, types of Variation, and the reasons behind variation.

2. Types of data :

• Variable Data : When a record is made of an actual measured quality characteristics, the quality is said to be in Variables.  e.g. 25.219 mm, 25 °K, 55 kg.
•  Attribute Data : Number of articles failing to specify requirements, is said to be Attribute Data. e.g. Cracks in sheet, Assembly defect of PCB.

3. Frequency Distribution, and use of it. SQC Tools such as Frequency Histogram, Bar Chart, Frequency Polygon, Ogive Curve, etc.

4. Central Tendency, ways for expressing Central Tendency such as Mean, Median, Mode. Concept of Dispersion, its measures such as Range, Standard Deviation, and Variance. ND curve and its Characteristics.

5. Concept of SQC, Objectives, Importance, and Advantages of SQC. Control charts, and its classification - Variable Data (X-bar and R chart)

                      - Attribute Data (P-chart and C-chart)

7.2 Acceptance Sampling :

Important points discussed in this Topic :

6. Acceptance sampling uses statistical sampling to determine whether to accept or reject a production lot of material. It has been a common quality control technique used in industry. It is usually done as products leaves the factory, or in some cases even within the factory.

Need of Sampling, Merits and Demerits of Acceptance Sampling over 100% Inspection.

7. Methods of Sampling such as :

• Single Sampling Plan :

• Double Sampling Plan :

• Multiple Sampling Plan :

7. Operating Characteristics Curve (OC Curve), Ideal and Actual OC curve.

8. AOQL and AOQ :

AOQL : Average Outgoing Quality Limit. The AOQL of a sampling plan is maximum value on the AOQ curve. It is applicable for defective units, defects per unit, and defects per quantity.

AOQ : The expected average quality level of an outgoing product for a given value of incoming product quality. AOQ plot represents the relationship between the quality of incoming and outgoing materials.

9. Process Capability of Machine :
Methods to find Process Capability, Different possibilities of Process Capability, its Calculations.

10. Comparison between :

• Defect and Defective
• Single Sampling Plan and Double Sampling Plan
• Single Sampling Plan and Multiple Sampling Plan
• P-chart and C-chart
• Attribute Inspection and Variable Inspection
• Control chart for Attributes and Control chart for Variables