QHT1 - CHAPTER 10

two basic questions about the process variations:

1. Are the variations random? If nonrandom variations are present, the process is considered to be unstable. Corrective action will need to be taken to improve the process by eliminating the causes of nonrandomness and achieve a stable process. 2. Given a stable process, is the inherent variability of process output within a range that conforms to performance criteria? This involves assessment of a process's capability to meet standards. If a process is not capable, this situation will need to be addressed.

Managerial Considerations Concerning Control Charts

1. At what points in the process to use control charts. 2. What size samples to take. 3. What type of control chart to use (i.e., variables or attribute).

The basic issues of inspection are:

1. How much to inspect and how often. 2. At what points in the process inspection should occur. 3. Whether to inspect in a centralized or on-site location. 4. Whether to inspect attributes (i.e., count the number of times something occurs) or variables (i.e., measure the value of a characteristic).

Inspection

Appraisal of goods or services.

Attributes

Attributes Generate data that are counted. (e.g., the number of defective parts in a sample, the number of calls per day);

Variables

Generate data that are measured. usually on a continuous scale (e.g., amount of time needed to complete a task, length or width of a part).

Inspection point 4- Before an irreversible process.

In many cases, items can be reworked up to a certain point; beyond that point they cannot. For example, pottery can be reworked prior to firing. After that, defective pottery must be discarded or sold as seconds at a lower price.

Assignable variation

In process output, a variation whose cause can be identified. A nonrandom variation.

acceptance sampling

Inspection before and after production

Control Charts for Variables

Mean and range charts are used to monitor variables. Control charts for means monitor the central tendency of a process, and range charts monitor the dispersion of a process.

Process variability

Natural or inherent variability in a process.

Random variation

Natural variation in the output of a process, created by countless minor factors.

Traditional view:

The amount of inspection is optimal when the sum of the costs of inspection and passing defectives is minimized

Central limit theorem

The distribution of sample averages tends to be normal regardless of the shape of the process distribution.

Control limits

The dividing lines between random and nonrandom deviations from the mean of the distribution.

Process capability

The inherent variability of process output relative to the variation allowed by the design specification.

Inspection point 3 - Before a costly operation.

The point is to not waste costly labor or machine time on items that are already defective.

Control charts for means monitor

the central tendency of a process,

range charts monitor

the dispersion of a process.

The purpose of inspection

to provide information on the degree to which items conform to a standard

To determine initial control limits, one can use the following procedure:

1. Obtain 20 to 25 samples. Compute the appropriate sample statistic(s) for each sample (e.g., mean). 2. Establish preliminary control limits using the formulas. 3. Determine if any points fall outside the control limits. 4. If you find no out-of-control signals, assume that the process is in control. If not, investigate and correct assignable causes of variation. Then resume the process and collect another set of observations upon which control limits can be based. 5. Plot the data on the control chart and check for out-of-control signals.

Quality control

A process that evaluates output relative to a standard and takes corrective action when output doesn't meet standards.

Quality of conformance

A product or service conforms to specifications.

Specifications

A range of acceptable values established by engineering design or customer requirements.

Run test

A test for patterns in a sequence.

Sampling distribution

A theoretical distribution of sample statistics.

Type II error

Concluding a process is in control when it is not. (consumer's risk, not found before passing on to consumer.)

Type I error

Concluding a process is not in control when it actually is. (producer's risk)

c-chart

Control chart for attributes, used to monitor the number of defects per unit.

p-chart

Control chart for attributes, used to monitor the proportion of defective items in a process.

Range control chart

Control chart used to monitor process dispersion.

Inspection point 2 - Finished products. Customer satisfaction and the firm's

Customer satisfaction and the firm's image are at stake here, and repairing or replacing products in the field is usually much more costly than doing it at the factory. Likewise, the seller is usually responsible for shipping costs on returns, and payments for goods or service may be held up pending delivery of satisfactory goods or remedial service. Well-designed processes, products and services, quality at the source, and process monitoring can reduce or eliminate the need for inspection.

Effective control requires the following steps:

Define. The first step means to define in sufficient detail what is to be controlled. It is not enough, for example, to simply refer to a painted surface. The paint can have a number of important characteristics such as its thickness, hardness, and resistance to fading or chipping. Different characteristics may require different approaches for control purposes. Measure. Only those characteristics that can be counted or measured are candidates for control. Thus, it is important to consider how measurement will be accomplished. Compare. There must be a standard of comparison that can be used to evaluate the measurements. This will relate to the level of quality being sought. Evaluate. Management must establish a definition of out of control. Even a process that is functioning as it should will not yield output that conforms exactly to a standard, simply because of the natural (i.e., random) variations inherent in all processes, manual or mechanical—a certain amount of variation is inevitable. The main task of quality control is to distinguish random from nonrandom variability, because nonrandom variability means that a process is out of control. Correct. When a process is judged out of control, corrective action must be taken. This involves uncovering the cause of nonrandom variability (e.g., worn equipment, incorrect methods, failure to follow specified procedures) and correcting it. Monitor results. To ensure that corrective action is effective, the output of a process must be monitored for a sufficient period of time to verify that the problem has been eliminated.

Inspection can be used as part of an effort to improve process yield.

One measure of process yield is the ratio of output of good product to the total output. Inspection at key points can help guide process improvement efforts to reduce the scrap rate and improve the overall process yield, and reduce or eliminate the need for inspection.

Inspection point 5.-Before a covering process.

Painting, plating, and assemblies often mask defects.

statistical process control

Quality control efforts that occur during production

Run

Sequence of observations with a certain characteristic.

Statistical process control (SPC)

Statistical evaluation of the output of a process.

Inspection point 1. Raw materials and purchased parts. 2. Finished products. Customer satisfaction and the firm's image are at stake here, and repairing or replacing products in the field is usually much more costly than doing it at the factory. Likewise, the seller is usually responsible for shipping costs on returns, and payments for goods or service may be held up pending delivery of satisfactory goods or remedial service. Well-designed processes, products and services, quality at the source, and process monitoring can reduce or eliminate the need for inspection. 3. Before a costly operation. The point is to not waste costly labor or machine time on items that are already defective. 4. Before an irreversible process. In many cases, items can be reworked up to a certain point; beyond that point they cannot. For example, pottery can be reworked prior to firing. After that, defective pottery must be discarded or sold as seconds at a lower price. 5. Before a covering process. Painting, plating, and assemblies often mask defects.

There is little sense in paying for goods that do not meet quality standards and in expending time and effort on material that is bad to begin with. Supplier certification programs can reduce or eliminate the need for inspection.

In the service sector, inspection points are

incoming purchased materials and supplies, personnel, service interfaces (e.g., service counter), and outgoing completed work (e.g., repaired appliances).

process control

monitoring during the production process

Control limits are

statistical limits that reflect the extent to which sample statistics such as means and ranges can vary due to randomness alone.