Step

Description

Process Capability Calculations

The following example provides you with a step by step procedure to review how to calculate the Process Capability.

Please do the following to do the process capability on a sample file.

a- Start MS Excel

b- Select the [2] Case b- X Moving Range Chart option from the Cases… option from the SQC menu

c- Click the OK button

d- Click the Finish button

e- Select the Capabilities Worksheet.

USL and LSL Limits

Let’s assume the following setup

Upper Specification Limit = USL = 73.00

Lower Specification Limit = LSL = 71.00

Please do the following to se the program limits.

a- Select the Analysis è General Setup… option from the SQC Menu

b- Make sure you enter the USL and LSL correctly (Note USL is the 1^st entry)

c- Review the results in the Capabilities worksheet

Calculation Options

There are three important parameters to select to display the process capability

Option	Description	Recommended
A	Calculation type	Cpk for all Ppk for auto Industry Six Sigma for Six Sigma
B	Use Average or Median	Use Average unless specified by customer
C	Standard Deviation Calculation Method	Use STD Sample

Options A – Calculation Type

There are four options used in this calculation

[1] Cpk: Cp and Cpk, commonly referred to as process capability indices, are used to define the ability of a process to produce a product that meets requirements. In the Cpk calculation, the standard deviation is based on subgroups of the data using subgroups ranges, standard deviations or moving ranges. Normally is calculated using Sigma (R_Ave/d2)

[2] PpK

In the Pp and Ppk calculations, the standard deviation used in the denominator is based on all of the data evaluated as one sample, without regard to any sub-grouping. The calculation is based on Standard Deviation of Data

[3] Six Sigma [short Term]

[4] Six Sigma [Long Term]

Options B – Average or Median

Median: The median is the number in the middle of a set of numbers; that is, half the numbers have values that are greater than the median, and half have values that are less.

Average: average (arithmetic mean) of a set of numbers. (Recommended)

Option C – Standard Deviation Method

There are four methods available to calculate the Sigma or standard deviation of the data. Normally option 3 is used.

(1) Standard Deviation Sample

The Sigma is calculated based on standard deviation based on a sample. The standard deviation is a measure of how widely values are dispersed from the average value (the mean).

Remarks

MS Excel STDEVA is used
STDEVA assumes that its arguments are a sample of the population. If your data represents the entire population, you must compute the standard deviation using STDEVPA.
Arguments that contain TRUE evaluate as 1; arguments that contain text or FALSE evaluate as 0 (zero). If the calculation must not include text or logical values, use the STDEV worksheet function instead.
The standard deviation is calculated using the "nonbiased" or "n-1" method.
STDEVA uses the following formula:

(2) Standard Deviation

The Sigma is calculated based on the standard deviation of the entire population given as arguments, including text and logical values. The standard deviation is a measure of how widely values are dispersed from the average value (the mean).

Remarks

Use MS Excel STDEVPA Function.
STDEVPA assumes that its arguments are the entire population. If your data represents a sample of the population, you must compute the standard deviation using STDEVA.
Arguments that contain TRUE evaluate as 1; arguments that contain text or FALSE evaluate as 0 (zero). If the calculation must not include text or logical values, use the STDEVP worksheet function instead.
For large sample sizes, STDEVA and STDEVPA return approximately equal values.
The standard deviation is calculated using the "biased" or "n" method.
STDEVPA uses the following formula:

(3) Standard Deviation form X-Chart

The Sigma is calculated from Average of Range divided by d2 constant.

(4) Standard Deviation from X-Bar Chart

The Sigma is calculated from Average of Range divided by d2 constant.

The data must be collected and stored in a spreadsheet similar to the spreadsheet shown here. There are few important parameters that you need to identify.

1- The number of data points ( In this example there are 5000 data points)

2- The number of decimal points that you would like to present you data ( In this example 5 decimal place is chosen)

3- What is the Upper Specification Limit or USL ( In this Example 0.003 is chosen). This is optional

4- What is the Lower Specification Limit or LSL ( In this Example -0.003 is chosen). This is optional.

Save the spreadsheet.

Please do the following steps:

1- Select the data ( Click on the first cell and then press the SHIFT KEY and click the last cell)

2- Choose New... option from the SQC menu

3- Click the Next button

4- Make sure you have selected the No Header Row option and click the Next button.

Please do the following steps:

1- Make sure you select the Individual option from the Control Charts pull down menu.

2- Click on the Properties button on the Analysis box and enter the following

Check Box to use USL

0.003 for the USL

Check box to use LSL

-0.003 for the LSL

as shown below.

You can decide on the Analysis type or the Standard Deviation Method as well in this screen.

3- Click the OK button

4- Click the Next button

5- Click the Finish button.

Please do the following steps:

1- Chose the Setup... option from the SQC menu.

2- Enter 5 as the number of decimal place and click the OK button.

Please do the following steps:

1- Chose the Report... option from the SQC menu. Make sure you select the Histogram and Process Capability options

2- Select the Save Report as MS Word and make sure you type a correct path and file name.

Example: C:\my_report or C:\SQC_Results\Capability

Make sure that there are no spaces in the folder or file name.

3- Click the Print button.

4- Using MS Explorer open the MS word file that you have created.

The SQC for Excel program uses a MS word template to generate the report. The file is called SQC Report.DOT and is located in the C:\Program Files\BaRaN Systems LLC\SQC for Excel 2002. Wrong modification to this file might result in problem in generating the correct report. Therefore make sure you make a backup of this file for future use.

Please do the following steps to modify the MS Word template.

1- Use MS Explorer and find the SQC Report.dot file

2- Click on the file name with the RIGHT MOUSE and then select the Open option.

3- Select the Options... option from the Tools menu and then click on the View tab.

4- Make sure that the Bookmarks option is selected.

5- Select the Bookmarks option from the View menu.

Please note that for each report type in the SQC for Excel, there is an equivalent book mark. Selecting the bookmark and click on the Go To button will move the MS Word cursor to the location of the bookmark.

You can modify the SQC Report.DOT file as much as you like as long as

A- The file name is kept as SQC Report.DOT

B- It is stored where SQC for Excel Program resides.

C- None of the bookmarks are not deleted ( They can be in different locations)

6- For example, the file is modified and saved to have the Capability and Histogram shown.

Lots of parameters are set to generate the reports, you can save all of the for the next time you are going to do the similar job.

Please do the following:

1- Select the Setup... from the SQC menu

2- Click on the Save button and select an Un-Used User Case ( Case 6 in this example) and click the OK button.

3- Type in a name that you can recall next time and click the OK button.

4- Next time you want to do the analysis, collect your data, and select New.. from SQC menu

5- Make sure you select the case that you have entered as shown below.

6- Click the Finish button to get your results.