TYPICAL PHASE II COMPUTER SESSION


This exercise section assumes that the subdirectories setup is exactly the same as outlined above. If not, replace the names of the directories where your Minitab and SQC software are located for MINTAB and SQC throughout the instructions.

MINITAB

For Phase II, there will be data consisting of 25 subgroups of 5 each (125 pieces total). Enter the data with Minitab. From there an ASCII data file can be created for use with the SQC software.

To use the Minitab software,

[CD\MINITAB]

Get into Minitab Directory

[Minitab]

Get into Minitab software

Press any key

Continue.

[Read C1-C3]

Creates three columns (one for each datum in the subgroup )

Enter the first row of data by typing in the first value, space, second value, space, and third value, and then press Enter. The second row of data is entered the same way as the first row. Repeat this procedure for each of the rows. Minitab will display messages on the screen if an entry error is made.

After the data have been correctly entered, type:

[END]

Gets out of the Minitab data insert mode.

Minitab will prompt '25 rows'. Check the data for errors!

[Print C1-C3]

Prints the three columns on the screen.

If there are errors, Minitab will allow for editing (see manual), or start the session over. In the long run, it is easier to learn editing techniques.

Once the data are correct, get a statistical summary of them.

[Describe C1-C3]

Statistical description of the data.

[Histogram C1-C3]

Histogram of the data.

[Dotplot C1-C3;]

Individual column means and range plotted

[Same.]

next to each other with the same scale.

The dotplot command plots the data of the individual columns on the same scale if entered exactly as above. The Minitab software requires a subcommand to do this. Placing a semicolon after the first command line tells the software to expect a subcommand. Placing a period after the subcommand tells the software that the subcommand is completed. Check the data for one row that is different from the others. The difference may indicate unusual variation or a process stream.

At this point, the data have been checked and reviewed. Create an output file to save the data collected on the tryout. Naming and storing the output file is very important. To output the data file for Minitab, use the following:

[Save 'filename']

Saves the Minitab file.

where filename is the name of the file without an extension. Next, create an ASCII data file for use with the SQC package. At the MTB> prompt, do the following:

[Write 'filename' C1-C3]

Creates ASCII Data file.

The Minitab software will put an extension of .DAT onto the file. The software requires that the filename be enclosed within single quotes.

This is the end of Minitab use for the time being. At the MTB> prompt, enter the following:

[STOP]

Ends the Minitab session.

The computer should then prompt either C:> or C:\Minitab> depending on the setup. For good recordkeeping practice, it is advantageous to put a three-line title in the ASCII (SQC) data file. This can be accomplished with any line editor package. If you do not have access to one, you may also use Edlin.

If you are unfamiliar with Edlin or line editors, you may elect to enter a title from within the SQC software. There are two drawbacks to this method. First, every time that the data file is used with SQC, the title will have to be manually inserted. Second, the title and comments will not remain with the data file, so in the future, there may be difficulty identifying where the data file came from. It is strongly recommended that comments and titles be entered into the data file.

SQC SESSION (VARIABLE DATA)

For those who have created the SQC.BAT file and wish to work with it, enter the following:

[SQC]

Invokes the SQC BATCH file.

The computer will automatically transfer to the SQC directory and start the software.

If there is no SQC.BAT file, do the following:

[CD\SQC]

Puts control into the SQC directory.

[SQC]

Invokes the SQC software.

In both cases the computer will show the SQC title page:

Press Enter.

Continues on to the Main Menu.

Read the data file by highlighting the second line down 'Input Data from a File'.

Press Enter

Allows data file input.

The software will prompt for the data file name. The data file is stored in the MINITAB directory. At the prompt, type:

[C:\MINITAB\filename.DAT]

filename is the name of the ASCII data file created with Minitab.

SQC will report that 125 samples have been read and will print the title from the data file. If there is no title in the data file, SQC will report, 'No Title Found'. You can enter a title by going into the 'Charting and Graphing Parameters Menu' found on the main menu and then selecting 'Enter Three-Line Title' from that menu.

After reading the data and titling (if necessary), check the data for abnormalities. Return to the main menu and select the 'Formatted Report (Stats + Hist)' command

Press Enter

Displays a histogram and statistics block for current data file.

Check the statistics mean and standard deviation; also look for nonzero skewness or kurtosis and excessive range. Check the histogram for points well outside the expected distribution. Any of the above indicate either process problems or incorrect data entry. If there are problems, find the cause and correct them now.

Assuming that all is correct, continue on. First, it is necessary to toggle some flags in the SQC software before obtaining output. In the Main Menu, toggle the capability flag by highlighting the fourth line down:

Press Enter

Toggles the capability flag.

Next, drop to the sixth line down, 'Specification Limits':

Press Enter

The software prompts for the Lower (specification) Limit.

Enter the lower limit and press Enter

The software will prompt for the nominal value.

If the nominal (target) is unknown, press Enter. The software will assume that the center of the limits is the target value.

Enter the target value and press Enter

The software will prompt for the upper limit.

Enter the upper limit and press Enter

The software will display the current specification limits.

Press Enter

Continue.

Select the 'Additional Charts and Graphs Menu' line, press Enter

Gains access to the subcommand menu.

Highlight the 6th line down 'Toggle Xbar Histogram Flag'

Toggles the flag for the software to display a histogram at the end of each

Press Enter

Xbar chart.

Press Esc

Returns to the main menu.

Highlight the 'Statistical Parameters Menu', press Enter

Gains access to the subcommand menu.

Select 'Set the Range Value for Estimation' option and press Enter

Allows user to change the estimated range reported in the stats block.

For the Phase II session, the process is expected to be able to be ±4 ƒ within the blueprint limits. Changing the estimation range also tells the software to calculate the Cp value based on the new standard deviation level.

Press Esc

Returns software to the main menu.

Highlight the 'Charting and Graphing Parameters Menu' line and press Enter

Gains access to the subcommand menu.

Highlight the top line, press Enter

Prompts for the subgroup size .

[5]

Tells the software to use subgroups of size 5

Press Esc

Returns to the main menu.

Highlight the 'Formatted Report (Stat and Hist)' option and press Enter

Statistics block.

The software reports the data file statistics, title, high and low data values, data range, sample size, and a ±4 ƒ estimate for high, low, and range. It will also display Cp and Cpk values.

SQC MACHINE ACCEPTANCE DATA

Typical Phase II Tryout Scenario

Sample size

125

Actual

EST 99.99%

Limits

Target

16.5000

Low

16.1915

16.0293

16.0000

Average

16.5143

High

16.8073

16.9992

17.0000

Std.Dev.

0.120

Range

.6158

.9699

1.0000

Skewness

-0.0416

% < Low limit.

.00

.00

 

Kurtosis

-0.3599

% > High limit.

.00

.00

 

Normality test made

 

% out of range.

.00

.00

 

Normality assumed

         

Subgroup size = 5

         
   

Cp

1.0310

   
   

Cpk

1.3398

   

The mathematical equations used to calculate all the above statistics have already been discussed either in this volume or Volume III.

Press Enter

Displays the data histogram.

On most computers, the pause button or Control and S combination will stop the histogram from scrolling. If nothing seems wrong with the data, continue.

Press Enter

Continue.

Down two lines from the Stats/Histogram command on the main menu is the Xbar chart. Highlight this section:

Press Enter

The computer reports the UCL, LCL, Grand mean, Group size, Title, and Xbar chart.

Press Enter

Continue.

Two different out-of-control flags can come up. The first looks like "< = = RS***" and represents a data run. It will appear when too many continuous data set means lie on one side of the grand average. The other flag looks like "< = = CL***" and represents an individual subgroup average outside the control limit. Both flags indicate that the data have gone out of control. Caution should be exercised when the run sum (RS) flag is shown. Seven points in a row is the definition of a run. SQC uses a variable number based on the sample size and probability theory. SQC also uses a 99% confidence level, but most companies use the standard of 95% confidence. Use your discretion when interpreting the data.

If the run flag is distracting, it can be shut off within the 'Additional Charts and Graphs Menu'. Doing so may also affect the flagged out-of-control message in the stats block. To turn off the run sums flag, highlight the 'Additional Charts and Graphs Menu' and do as follows :

Press Enter

Gains access to the subcommand Menu.

Highlight the 'Toggle Discrete

Shuts off the run sums flag.

Run Sums Flag' line, press Enter

 

The software will no longer flag runs or trends. Be certain that you watch carefully for these special variation signals.

Press the Esc key

Returns to the main menu.

The next line down in the main menu is the Range Chart. Highlight this line:

press Enter

Displays the data range chart.

The SQC software will report a range chart for the data using the same subgroup size as the Xbar chart. Again, you will see the RS and CL flags mentioned above if the range is out of control.

When you have finished viewing the data and are satisfied with the results, create an output file to record the session.

There is one more flag to set to format the output file successfully. In the 'Charting and Graphing Parameters Menu' (on the Main Menu) is a 'Toggle the Page Flag' command. Highlight it

Press Enter

Edits the output file to place one report per page when printed from the output file. The page Flag should light at the screen bottom.

Press the Esc key

Returns to the main menu.

Highlight the 'Route Report Output to File/Printer' command:

press Enter

Choice of output.

SQC will prompt with "Enter filename or 'Printer'." Enter a filename to create a permanent record of the SQC session. The output file can always be printed later for hard copies.

The output file naming is important. You already have a file with an .OUT extension pertaining to the data file in the Minitab directory. The best convention to use here for naming would be to use an .SQC extension with the same filename you have used all along. Though an .SQC extension is not critical (you may use whatever you wish), consistency is important; you already have the extensions .MTW, .OUT, and .DAT associated with this data. The remainder of this section will assume that you have used the same naming conventions.

The output file should consist of a formatted report (stats and histogram), Xbar chart, Range chart, and an individuals chart. To get these into the output file, highlight the command lines (one at a time) and press Enter. The computer screen will go blank while the data are downloaded into the output file.

After entering the four separate commands, highlight the bottom command:

press Enter

Terminates the SQC session.

You now have a file of the SQC session in the computer. You can view the file with any line editor or print a hard copy for review.




Six Sigma and Beyond. Statistical Process Control (Vol. 4)
Six Sigma and Beyond: Statistical Process Control, Volume IV
ISBN: 1574443135
EAN: 2147483647
Year: 2003
Pages: 181
Authors: D.H. Stamatis

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