Quality Control Acceptance Sampling Systems

Lot-by-Lot Acceptance Sampling Plans for Attributes Devised in 1942 at Bell Labs Later became MIL-STD-105E ISO-2859 ANSI/ASQ Z1.4 Wording & terminology changes Additional tables added

ANSI/ASQ Z1.4 This standard is applicable to: End items Components and raw materials Operations Materials in process Supplies in storage Maintenance operations Data or records

ANSI/ASQ Z1.4 Intended to be used for a continuing series of lots, but may be designed for isolated lots. Standard provides for single, double, and multiple sampling plans. Provision is also provided for normal, tightened, or reduced inspection. Plan is specified by the AQL, and samplesize code.

ANSI/ASQ Z1.4 Normal inspection is used at the start of inspection with changes being a function of recent quality history. Tightened inspection: Generally used when producer s recent quality history has deteriorated.

ANSI/ASQ Z1.4 Reduced inspection: Used when the producer s recent quality history has been exceptionally good. Decision concerning what type of plan to use is left to the responsible authority. Nonconformities are classified such as critical, major

ANSI/ASQ Z1.4

Sample Size Code Letters

Comparison of Inspection levels 100 95 = 0.05 80 Percent of Lots Accepted (100Pa) 60 40 III II I = 1/2 n II = n III = 2n I 20 100 =10 0 2 4 6 8 10 Percent Nonconforming (100P a )

ANSI/ASQ Z1.4 AQL The AQL is the most important part of the standard since the AQL and sample size dictate the plan used. AQL is defined as the maximum percent nonconforming that can be considered satisfactory as a process average. Satisfactory = Producer s risk, α Usually 0.05

ANSI/ASQ Z1.4 AQL AQL is specified by contract or responsible authority. AQL may be determine from historical data, empirical judgment, engineering information, experimentation, producer s capability, consumer s requirements.

Sample Size ANSI/ASQ Z1.4 Determined by lot size and inspection level Use of sample-size code Inspection level is determined by the responsible authority.

Sample Size ANSI/ASQ Z1.4 Different levels of inspection provide approximately the same protection to the producer, but different protections to the consumer. Plan provides for special levels to be used where relatively small sample sizes are necessary and large sampling risks must be tolerated. (S-1, S-2, S-3, S-4)

Implementation ANSI/ASQ Z1.4 Determine lot size Determine inspection level Find sample-size code letter in table Determine AQL

Implementation cont d. ANSI/ASQ Z1.4 Determine type of sampling plan Find sampling plan in appropriate table Start with normal inspection and change to tightened or reduced based on switching rules

AQL What an AQL means is that as long as a supplier maintains his/her process average (% defective) at the assigned AQL or lower, there is a very high probability that shipments from that supplier, when inspected using ASQ Z1.4 sampling plans, will be accepted.

AQL By the same token, there is a very high probability that shipments, when inspected using ASQ Z1.4 sampling plans, will be rejected if a supplier's process average (% defective) remains higher than the assigned AQL.

ANSI/ASQ Z1.4 Single sampling plans Start by identification of AQL, lot size, inspection level, and type of sampling plan. Locate appropriate plan parameters in tables. Double and Multiple Sampling Similar procedures as single sampling

ANSI/ASQ Z1.4 Normal, Tightened, and Reduced Inspection Start with normal inspection Switching Procedures Normal to tightened Institute when 2 out of 5 consecutive lots not accepted on original inspection Tightened to normal Institute when 5 consecutive lots accepted If not, then discontinue inspection under this plan

ANSI/ASQ Z1.4 Switching Procedures Normal to reduced Institute when: (all of the following) Preceding 10 lots on normal inspection have been accepted Total nonconforming in preceding 10 samples is less than number identified in Table 10-5. Production is at a steady rate Reduced inspection is considered desirable by responsible authority

ANSI/ASQ Z1.4 Switching Procedures Reduced to normal Institute when: (any of the following) A lot is not accepted Sampling procedure terminates with neither acceptance or rejection criteria have been met Production is irregular or delayed Other conditions

ANSI/ASQ Z1.4 Standard is designed for use where units of product are produced in a continuing series of lots or batches. If a sampling plan is desirable for a lot or batch of an isolated nature, it should be chosen based on the Limiting Quality (LQ) and consumer s risk, b. These tables are included in the standard, but not the textbook. Or use ANSI/ASQ Standard Q3

ANSI/ASQ Standard Q3 Standard is used for inspection of isolated lots by attributes. Provides indexed tables by Limiting Quality, LQ.

ANSI/ASQ Standard Q3 Two Schemes: 1. Used for lots that are isolated or mixed or that have an unknown history as far as all parties know. Lot size and LQ must be known. Nominal values of the LQ are based on b=0.10

ANSI/ASQ Standard Q3 Two Schemes: 2. Used when a vendor is producing a continuous stream of lots and sends one or a few to a customer who will consider them as isolated lots. Occurs frequently when purchasing small quantities of raw materials.