Ch.21 Statistical Process Control

14. What are attribute Control Chart?

Attributes control charts require only a count of defective parts or services, rather than precise and careful measurements. The simplicity of this approach is offset by the requirement for more sample data needed for process control. Attributes charts are used for count data. For attributes control charts, if every item is in one of two categories, such as good or bad, "defectives" are counted.

2. What is statistical process control (SPC)?

The application of statistical techniques for measuring and analyzing ( and hopefully reducing) the variation in processes.

3. What is statistical quality control?

The application of statistical techniques for measuring and improving the quality of processes.

18. What is c Control Chart?

The c-chart measures the number of defects within a sample. When defects are counted and the sample size is constant, the c-chart may be used instead of the u-chart. This is relevant for processes that have a continuous flow or where there are many different potential sources of variation or deficiencies. Note that this chart is slightly simpler to use than the u-chart because the number of defects is plotted rather than the fraction of the sample.

6. What is common cause variation?

The common causes of variation are those that are inherent to the process and generally are not controllable by process operators. Common cause variation is also known as natural variation and refers to the many sources of variation within a process. Common causes reside in processes within statistical control, and can be characterized by location (process average), spread (piece-to-piece variability), and shape (distribution) for predictability.

4. What are the objectives and benefits of SPC?

* Monitoring processes in real time * Identifying whether processes are operating as expected * Identifying whether processes have changed and corrective actions may be required * Making statistical valid decisions * Centering the process * Determining when and when not to take action on the process * Determining the type of action to take (that is, actions to eliminate special causes , actions to improve the overall process) * Quantifying and reducing variation * Improving understanding of products and processes * Improving product and process design * Monitoring continual improvement and confirming that changes were effective

1. What is control process?

A Feedback loop through which we measure actual performance, compared it with a standard, and act on the difference.

13. What is Median Control Chart?

Median charts plot the median of the sample rather than the average. This chart is often used when outliers are expected. All data points in the sample are plot-ted, and the user connects the middle point in successive samples. (pg395)

5. What are the causes types of process variation?

Special & Common

8. What is special cause variation?

Special causes of variation include unusual events that the operator, when prop-erly alerted, can usually remove or adjust. Special causes are sometimes called assignable causes. Unless all the special causes of variation are identified and miti-gated, the process output will be unpredictably influenced, with random results

16. What is np Control Chart?

The np-chart measures the number of rejected items in a sample with an integer rather than a proportion. The np-chart is most useful when sample sizes are constant and the integer number is more meaningful and relevant than the proportional decimal amount.If defectives are being counted and the sample size remains constant, the np chart can be used instead of the p-chart. (pg.400)

15. What is p Control Chart?

The p-chart measures the proportion of defective parts or pieces within the group under review. This could be for a single characteristic or multiple characteristics. Pieces either conform or are rejected. The rejected portion is expressed as a decimal fraction of the sample size.The p-chart is used to chart binary data where each item is in one of two cate-gories. This would be the appropriate chart for plotting numbers of defectives, for instance. In the following example, each blood sample is in one of two categories, so the p-chart is appropriate, although neither category is defective

17. What is u Control Chart?

The u-chart is appropriate to use when defects rather than defectives are counted. The u-chart measures the number of defects or problems on a per unit basis. The example in Figure 21.16 shows the results of inspecting panes of glass in which defects include bubbles, scratches, chips, inclusions, waves, and dips. The number of defects is counted and recorded for each sample, and the fraction (# defects ÷ sample size) is calculated and plotted

11. What is X bar and S Control Chart?

This chart shows the average and the sigma for the process values. Unlike the range, which is calculated by subtracting the smallest value in the subgroup from the largest, the sigma, or standard deviation, value is calculated from all values in the subgroup. When the subgroup is smaller (that is, five or fewer), the range and sigma charts yield similar results. The increase or decrease in sigma provides a more precise indicator of process change requiring further investigation.The X bar and s control chart is very similar to the X bar and R chart except that each value in the range row is replaced by the sample standard deviation s

10. What is X bar and R Control Chart?

This control chart shows how the process average changes, along with corresponding changes in process variation. Both average and variation must be con-trolled