4 BELL SYSTEM TECHNICAL JOURNAL 



Inspection Procedures 



Two distinct methods of inspection are employed — single sampling and 

 double sampling. In single sampling, only one sample is permitted before 

 a decision is reached regarding the disposition of the lot, and the acceptance 

 criterion is expressed as an allowable defect number, c. In double sampling, 

 a second sample is permitted if the first fails, and two allowable defect 

 numbers are used — the first, Ci, applying to the observed number of defects 

 for the first sample alone, and the second, C2, applying to the observed num- 



Inspect a first sample 



OF rii PIECES. 



If THE NUMBER OF DEFECTS 

 FOUND IN THE FIRST SAMPLE 



DOES NOT 

 EXCEED C| 



EXCEEDS C,, 



BUT DOES NOT 



EXCEED 02 



EXCEEDS C2 



INSPECT A SECOND SAMPLE 

 OF rig PIECES. 



If the NUMBER OF DEFECTS 

 FOUND IN THE FIRST AND 

 SECOND SAMPLES COMBINED 



DOES NOT 

 EXCEED Co 

 I 



EXCEEDS C2 



ACCEPT 

 THE 

 LOT 



INSPECT ALL THE PIECES IN THE REMAINDER 

 OF THE LOT, AND CORRECT OR REPLACE ALL 

 DEFECTIVE PIECES FOUND 



Fig. 1 — Double sampling inspection procedure 



ber of defects for the first and second samples combined. The specific 

 procedures assumed in the development of the tables are as follows: 



Single Sampling Inspection Procedure 



(a) Inspect a sample of n pieces. 



(b) If the number of defects found in the sample does not exceed c, accept 

 the lot. 



(c) If the number of defects found in the sample exceeds c, inspect all the 



pieces in the remainder of the lot. 



(d) Correct or replace all defective pieces found. 



Double Sampling Inspection Procedure 



(a) Inspect a first sample of ni pieces. 



(b) If the number of defects found in the first sample does not exceed ci, 

 accept the lot. 



