Plastics : Quick Reference to Sampling In Quality Control

Sampling In Quality Control:

Basic Definition:

Sampling is the process of selecting and testing a representative portion of a larger group or lot to make decisions about the entire group's quality.

Key Sampling Methods: 

Random Sampling:

Selection Method:
- Every item has equal chance
- No pattern in selection
- Unbiased representation
Example: Using random number generator

 Systematic Sampling:

Selection Pattern:
- Every Nth item
- Fixed interval
- Regular pattern
Example: Every 10th part from production

Stratified Sampling:

Group-Based:
- Divide into subgroups
- Sample from each group
- Proportional selection
Example: Sampling from different shifts

Common Sample Sizes:

Based on Lot Size:
Lot ≤280      →  Sample 32
Lot 281-500   →  Sample 50
Lot 501-1200  →  Sample 80
Lot 1201-3200 →  Sample 125
Lot >3200     →  Sample 200

Acceptance Criteria:

Critical Defects:   AQL 0.065%
Major Defects:      AQL 1.5%
Minor Defects:      AQL 2.5%

Basic Rules:

- Sample must be representative
- Selection must be random
- Size must be statistically valid
- Method must be consistent
- Documentation is essential

Common Mistakes to Avoid:

DON'T:
- Sample only easy-to-reach items
- Use damaged samples
- Mix different lots
- Change methods mid-process
- Skip documentation