Statistical Process Control

Cases and Problems

Logitech company – Problem

Day No of Defective

s

Day No of Defective

s

Day No of Defective

s

1 3 8 7 15 7

2 10 9 18 16 6

3 13 10 1 17 19

4 4 11 0 18 3

5 12 12 4 19 9

6 14 13 9 20 7

7 8 14 22

A sample of 50 disks per day is inspected

Case Question

Production manager suspects the variation was due to substitution of an inexperienced candidate instead of regular one on 3 days during the period.

Indigo airlines Problem

In Indigo aircraft's landing gear assembly the defects are detected as given in the table below

If the weightages given for the different classes of defects are A=10, B=5, and C=1. construct appropriate control chart for quality.

Plotting on a graph sheet.

Scale

Please note:

In end semester examination, if you are asked to draw/solve a control chart problem, please use 6 sigma limits only,(do not use 3 sigma control limit (1.5 sigma + 1.5 sigma) or you will loose marks).

Even if the question tells you to draw 3 sigma control limits, please understand that you are expected to draw 6 sigma control limits( 3sigma + 3 sigma).

Case study : Scharadin Hotels

National hotel chain started in 1957 by Milo Scharadin.

Highly reputable national hotel chain

Serve over 100 1ocations and are recognized for their customer service and quality.

They cater to both business and nonbusiness customers

A Problem with Quality

Scharadin Hotel in San Antonio, Texas, had recently been experiencing a large number of guest complaints due to billing errors.

Guest complaints ranged from extra charges, such as meals or services that were not purchased, to confusion for not being charged at all

The hotel staff often had to calm frustrated guests who were rushing to the airport and were aggravated that they were getting charged for items they had not purchased.

Staff meeting discussion

Larraine Scharadin, Milo Scharadin’s niece - recent business school graduate new computer system and software

Tim Coleman, head of MIS, defended the system

Tim claimed that a few hotel guests made an issue of a few random problems.

Scott Schultz, head of operations, was not so sure.

Scott said that he noticed that the number of complaints seem to have significantly increased since the new system was installed.

Audit of 50 random bills per day over the past 30 days.

Tim exclaimed: “Notice that the number of errors increases in the last third of the month. The computer system had been in place for the entire month so that can’t be the problem. Scott, it is probably the new employees you have on staff that are not enteringthe data properly.”

Scott quickly retaliated: “The employees are trained properly! Everyone knows the problem is the computer system!”

Case Questions

1. Set up three-sigma control limits with the given data.

2. Is the process in control? Why?

3. Based on your analysis do you think the problem is the new computer system or something else?

4. What advice would you give to Larraine based on the information that you have?

© 2006 Prentice Hall, Inc. S6 – 14

Patterns in Control Charts

Normal behavior. Process is “in control.”

Upper control limit

Target

Lower control limit

Figure S6.7

© 2006 Prentice Hall, Inc. S6 – 15

Upper control limit

Target

Lower control limit

Patterns in Control Charts

One plot out above (or below). Investigate for cause. Process is “out of control.”

Figure S6.7

© 2006 Prentice Hall, Inc. S6 – 16

Upper control limit

Target

Lower control limit

Patterns in Control Charts

Run of 5 above (or below) central line. Investigate for cause. Figure S6.7

© 2006 Prentice Hall, Inc. S6 – 17

Upper control limit

Target

Lower control limit

Patterns in Control Charts

Erratic behavior. Investigate.

Figure S6.7

Career in Quality

Six sigma Green Belt, Black Belt

Master black beltASQ or ISILead auditor

Thank you