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Lecture 13

# CTD 461 Lecture 13: CTD 461 Chapter 13 Notes Premium

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School
Department
Clothing, Textiles and Interior Design
Course
CTD 461
Professor
Ms.Davis
Semester
Spring

Description
CTD 461 Chapter 13 Notes Sampling A LOT Introduction • Production lot: product of same material, made by same process, and possibly worked on by same operators • How do we select for inspection audits and identify unwanted variation of the product? • When is a production lot unacceptable to a buyer? Sampling A Lot • Sampling: the process of selecting items from a production lot to evaluate in a product audit • Assumed to represent the whole lot • Based on: size of lot, acceptable level of quality, buyer/vendor relationship, defects that might occur • If sample drawn from lot so that each item has an equal chance of being selected then any problems should be present in the sample Sampling and Statistics • Universe: entire population being tested- the entire group about which you wish to obtain information (lot) ➢ In fabric production, this might refer to a 250,000-yard production run to fill an order for a fabric for a particular end use. ➢ If the fabric is woven, perhaps 50 looms, each weaving 5,000 yards, are involved • Sample: that portion of the universe which is selected for testing ➢ For the case above, if each loom produces five 1,000-yard rolls, and each roll is sampled once, then there will be 5x50 = 250 samples taken to characterize the production. ➢ Each samples consist of a 3-5 yard length taken from each roll • Specimen: a specific portion of a sample upon which a particular test is performed ➢ For example, to determine fabric breaking strength in the warp direction, five specimens measuring 6” by 4” are needed from each sample • To be sure that the results obtained by textile testing are truly characteristics of their universe, both the samples and specimens taken should be random and representative Types of Samples • Random: free of bias, each product in the lot (universe) has an equal chance of being selected for inspection regardless of its sequence of production, color, size, location in the boxes, or any other factor of production, packaging, or convenience ➢ Assign separate # of each item ➢ Randomly select a set of number and pull the product ➢ The number of sample products relates to the size of the lot and the type of inspection to be conducted • Representative: includes all colors, styles, sizes, and other planned variations in the product; not automatically random unless each item in the lot has an equal chance of being selected • Convenience sample: sample items were selected because they were easier to locate and select compared to other potential samples • Stratified sample: the lot is stratified into layers, each layer subdivided into cubes, if a cube is selected then all items in that cube will be tested, makes pulling the sample easier • Constant percentage sample: the same percentage is used regardless of lot size to determine the number of units to be sampled; not enough sample for small lots and too much for large lots • Systematic sample: selects units from equally occurring location or at equal intervals of time; useful for continuously operating production lines (same product for weeks) Example for Procuring A Sample • Specimens from fabric samples 1. Do not take specimens from near fabric selvages 2. Do not use the same yarns in the same test procedure 3. Mark and cut specimens on-grain, i.e., parallel and perpendicular to the selvages 4. Where appropriate, indicate fabric length (warp direction for wovens) with a mark 5. Do not place marks in critical test areas or where marks could alter test results 6. Always follow rules 1 through 5 Sampling Plan • How to select a reasonable number of products to represent the production unit or lot base on color, size, style, and other dimensions of importance • Acceptance sampling: the inspection and evaluation of sample products selected from production lots for predetermined and defined quality characteristics after the product has been produced • Military standard 105E: document basis for sampling and setting acceptable quality limits. Cancelled in 1995 • ANSI/ASQ Z1.4 – 1993 or ISO 2959 – 1999 Balance Sample • Balance sample is representative, costs, confidence in results • If sample is too low, low costs, results may not be valid • If sample is too high, costs increase greatly without a corresponding increase in confidence of results • Sampling plans: assume products are homogenous throughout lot • Tables are available to decide the number of units for a sampling plan Sampling Plans • Sampling plans are chosen based on: 1. Previous history of a supplier 2. How inspection is done: lot-by-lot, skip lot, continuous production basis 3. Perception of consumer detection of nonconforming product Ideal Sampling Plan • Samples represent fully and accurately the quality of the lot • Reject all lots worse than the standard • Accept all lots equal to or better than the standard • Possibility: ➢ Type 1 error or alpha, supplier risk: acceptable lots rejected ➢ Type 2 error or beta, consumer risk: inferior lots accepted ➢ Small sample size increases risk of type 1 or 2 error Inspection Levels • Identify the specific number of products pulled for the sample (sample size) • Normal inspection: is used when no evidence exists that the quality of the product submitted for evaluation is better or poorer than the specified quality level • Tightened inspection: used when product inspection makes it apparent that product quality is deteriorating, continues until a specified improvement in quality level or production is demonstrated, examines a larger # of products • Reduced inspection level: used when it is apparent that product quality is very good, requires smaller sample size, product consistent over long period of time Types of Defects • Defect: a nonconformance of a product with specific requirements, a feature that is else than what was specified; can depend on zone • Defective: a product with one or more defects • Critical defect: results in hazardous or unsafe conditions for the individual or prevents performance of a tactical function of a major end-use item (occupational apparel); defect makes it impossible to sell product for full price • Major defect: results in product failure or reduce potential use of product of intended purpose; effects appearance (stripes not match), or function and performance • Minor defect: not likely to reduce materially the usability of a product for its intended purpose • Patent defect: observed visually during examination • Latent defect: hidden defects, not apparent until material used or tested Establishing Acceptance Levels • Critical in understanding the point at which a sample’s lack of adherence to standards and specs is so extreme that the lot becomes unacceptable to the buyer
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