【正文】 ts in delivered goods. Initally, all materials from a supplier are inspected and batches of goods with any defective items are returned. Suppliers with good records can be certified and not subject to plete inspection subsequently. The traditional microeconomic view of quality control is that there is an optimum proportion of defective items. Trying to achieve greater quality than this optimum would substantially increase costs of inspection and reduce worker productivity. However, many panies have found that mitment to total quality control has substantial economic benefits that had been unappreciated in traditional approaches. Expenses associated with inventory, rework, scrap and warranties were reduced. Worker enthusiasm and mitment improved. Customers often appreciated higher quality work and would pay a premium for good quality. As a result, improved quality control became a petitive advantage. Of course, total quality control is difficult to apply, particular in construction. The unique nature of each facility, the variability in the workforce, the multitude of subcontractors and the cost of making necessary investments in education and procedures make programs of total quality control in construction difficult. Nevertheless, a mitment to improved quality even without endorsing the goal of zero defects can pay real dividends to organizations. Example 132: Experience with Quality Circles Quality circles represent a group of five to fifteen workers who meet on a frequent basis to identify, discuss and solve productivity and quality problems. A circle leader acts as liason between the workers in the group and upper levels of management. Appearing below are some examples of reported quality circle acplishments in construction: 1. On a highway project under construction by Taisei Corporation, it was found that the loss rate of readymixed concrete was too high. A quality circle posed of cement masons found out that the most important reason for this was due to an inaccurate checking method. By applying the circle39。 Example Operating Characteristic Curves Indicating Probability of Lot AcceptanceFor any bination of n and r, we can read off the value of g(p) for a given p from the corresponding OC curve. For example, n = 15 is specified in Figure 131. Then, for various values of r, we find: r=0r=0r=1r=1p=24%p=4%p=24%p=4%g(p) 2%g(p) 54%g(p) 10%g(p) 88%The producer39。s risk is the chance that an unacceptable lot is acceptable (ie. has less than the allowable number of defective items in the sample) due to a better than average quality in the sample. For example, suppose that a sample size of 15 is chosen with a trigger level for rejection of one item. With a four percent acceptable level and a greater than four percent defective fraction, the consumer39。s risk would be only five percent with this sampling plan. Example 134: Designing a Sampling Plan Suppose that an owner (or product consumer in the terminology of quality control) wishes to have zero defective items in a facility with 5,000 items of a particular kind. What would be the different amounts of consumer39。s risk is at most 1 = or twelve percent.In specifying the sampling plan implicit in the operating characteristic curve, the supplier and consumer of materials or work must agree on the levels of risk acceptable to themselves. If the lot is of acceptable quality, the supplier would like to minimize the chance or risk that a lot is rejected solely on the basis of a lower than average quality sample. Similarly, the consumer would like to minimize the risk of accepting under the sampling plan a deficient lot. In addition, both parties presumably would like to minimize the costs and delays associated with testing. Devising an acceptable sampling plan requires trade off the objectives of risk minimization among the parties involved and the cost of testing. Example 133: Acceptance probability calculation Suppose that the sample size is five (n=5) from a lot of one hundred items (N=100). The lot of materials is to be rejected if any of the five samples is defective (r = 0). In this case, the probability of acceptance as a function of the actual number of defective items can be puted by noting that for r = 0, only one term (x = 0) need be considered in Eq. (). Thus, for N = 100 and n = 5: For a two percent defective fraction (p = ), the resulting acceptance value is:Using the binomial approximation in Eq. (), the parable calculation would be:which is a difference of , or percent from the actual value of found above.If the acceptable defective proportion was two percent (so p1 = p2 = ), then the chance of an incorrect rejection (or producer39。s risk can be related to various points on an operating characteristic curve. Producer39。s risk) or that deficient lots might be incorrectly accepted (termed consumer39。 for example, ladder safety is covered by over 140 regulations. In cases of extreme nonpliance with standards, OSHA inspectors can stop work on a project. However, only a small fraction of construction sites are visited by OSHA inspectors and most construction site accidents are not caused by violations of existing standards. As a result, safety is largely the responsibility of the managers on site rather than that of public inspectors. While the multitude of participants involved in the construction process require the services of inspectors, it cannot be emphasized too strongly that inspectors are only a formal check on quality control. Quality control should be a primary objective for all the members of a project team. Managers should take responsibility for maintaining and improving quality control. Employee participation in quality control should be sought and rewarded, including the introduction of new ideas. Most important of all, quality improvement can serve as a catalyst for improved productivity. By suggesting new work methods, by avoiding rework, and