Asq Z1.4-2008 Pdf Free 32
ASQ Z1.4-2008: A Guide to Sampling by Attributes
Sampling by attributes is a method of quality control that involves inspecting a sample of items from a lot and classifying them as either conforming or nonconforming to a specification. The purpose of sampling by attributes is to estimate the proportion of nonconforming items in the lot and to decide whether to accept or reject the lot based on a predetermined acceptance criterion.
One of the most widely used standards for sampling by attributes is ANSI/ASQ Z1.4-2008, which is an American National Standard developed by the American Society for Quality (ASQ) and the American National Standards Institute (ANSI). This standard provides sampling plans and procedures for inspection by attributes based on the Acceptable Quality Limit (AQL), which is the maximum percentage of nonconforming items that can be considered acceptable for a given inspection level and lot size.
How to Use ANSI/ASQ Z1.4-2008
To use ANSI/ASQ Z1.4-2008, you need to follow these steps:
Determine the inspection level, which is a measure of the degree of inspection that you want to apply to the lot. There are three general inspection levels (I, II, and III) and four special inspection levels (S-1, S-2, S-3, and S-4). The higher the inspection level, the larger the sample size and the stricter the acceptance criterion. The inspection level can be chosen based on the importance of the product, the cost of inspection, the risk of defective items, and other factors.
Determine the AQL, which is the quality level that you are willing to accept for the lot. The AQL can be chosen based on the customer requirements, the industry standards, the regulatory requirements, and other factors. The standard provides AQL values ranging from 0.01% to 10% for different types of defects.
Determine the lot size, which is the number of items in the lot that you want to inspect. The lot size can be fixed or variable depending on the production process and the availability of data.
Find the appropriate sampling plan from the standard tables based on the inspection level, the AQL, and the lot size. The sampling plan consists of two parameters: the sample size (n), which is the number of items that you need to draw from the lot and inspect, and the acceptance number (c), which is the maximum number of nonconforming items that you can find in the sample and still accept the lot.
Draw a random sample of size n from the lot and inspect each item for conformity to the specification. Count the number of nonconforming items (x) in the sample.
Compare x with c and make a decision about the lot. If x c, accept the lot; if x > c, reject the lot.
Example of Using ANSI/ASQ Z1.4-2008
Suppose you want to inspect a lot of 500 widgets for a critical defect that affects their functionality. You decide to use a general inspection level II and an AQL of 0.65%. You follow these steps:
You look up Table I-A in [the standard] and find that for inspection level II and a lot size of 500, you need to use code letter H.
You look up Table II-A in [the standard] and find that for code letter H and an AQL of 0.65%, you need to use a sample size of 80 and an acceptance number of 0.
You draw a random sample of 80 widgets from the lot and inspect them for defects. You find that 2 widgets are nonconforming.
You compare 2 with 0 and decide to reject the lot because 2 > 0.
Advantages and Disadvantages of ANSI/ASQ Z1.4-2008
Some of the advantages of using ANSI/ASQ Z1.4-2008 are:
It is easy to use and understand, as it only requires counting and comparing numbers.
It is widely recognized and accepted by many industries and customers as a reliable method of quality control.
It provides flexibility in choosing the inspection level and the AQL based on the needs and preferences of the user.
It allows for switching between different sampling plans based on the quality history of the supplier or the process.
Some of the disadvantages of using ANSI/ASQ Z1.4-2008 are:
It does not account for the variability of the process or the measurement system, which may affect the accuracy and precision of the inspection results.
It does not provide a direct estimate of the process capability or the lot quality, which may be useful for continuous improvement and risk management.
It may result in either over-inspection or under-inspection depending on the choice of the inspection level and the AQL, which may affect the cost and effectiveness of the inspection.
It may not be suitable for some types of products or processes that require more sophisticated or customized sampling methods.
ANSI/ASQ Z1.4-2008 is a standard that provides sampling plans and procedures for inspection by attributes. It is a simple and widely used method of quality control that involves inspecting a sample of items from a lot and classifying them as either conforming or nonconforming to a specification. It is based on the Acceptable Quality Limit (AQL), which is the maximum percentage of nonconforming items that can be considered acceptable for a given inspection level and lot size. To use ANSI/ASQ Z1.4-2008, you need to determine the inspection level, the AQL, and the lot size, and then find the appropriate sampling plan from the standard tables. You then draw a random sample from the lot, inspect each item, count the number of nonconforming items, and compare it with the acceptance number to decide whether to accept or reject the lot. ANSI/ASQ Z1.4-2008 has some advantages and disadvantages that you need to consider before applying it to your quality control process.