Home / Statistical Tools / Analysis Tools / Cpk/Histogram / How-To

How-To¶

Quick Start¶

Create your first Cpk/Histogram in less than two minutes.

This example uses the Capability Analysis button with a single column of numeric data and two specification limits. Quantum XL draws the Normal curves for the data, marks the specification limits, and calculates the capability indices and PPM.

Two Ribbon Buttons

You can access this tool from two ribbon buttons: Histogram and Capability Analysis. The Histogram button starts with Display histogram bars checked and Display PDF curve unchecked. The Capability Analysis button starts the other way round: Display PDF curve checked and Display histogram bars unchecked. Both check boxes are on the Display Options tab of the dialog, so to see the bars and the curves together, check the one that is off before clicking Finish. Settings you saved with Save as Default Settings replace these starting values, separately for each button.

Goal¶

Run a capability analysis on diameter measurements with LSL = 9.5 and USL = 10.5.

Sample Data¶

Press Copy for Excel, then paste the table into a blank worksheet.

Diameter
9.82
10.05
9.97
10.12
9.91
10.08
9.88
10.15
9.95
10.03

Each row represents one diameter measurement. The 10 values run from 9.82 to 10.15, and the specification limits are 9.5 and 10.5.

Steps¶

Put the data in Excel

Press Copy for Excel above the table, click cell A1 in a blank worksheet, and press Ctrl+V. The header row lands in row 1 and the 10 data rows in rows 2 through 11.
Launch the analysis

From the Excel ribbon, select QXL Stat Tools → Analysis Tools → Capability Analysis.
Select your data

In the Data Selection window, select cells A1:A11 (the header row plus all 10 data rows). Make sure Data in Columns and First Row/Column is Header are selected, then click Next >.
Configure the analysis

In the Process Capability (Cpk) and Histogram dialog, on the Data tab:
- Measurement Columns: "Diameter" is checked (the first numeric column is checked for you)
- Subgroups: leave Constant size: at n = 1, so each value is one individual measurement
- Specification Limits: in the Diameter row, type 9.5 under LSL and 10.5 under USL, and leave Target empty
Click Finish to generate the chart.

Result¶

Quantum XL creates a worksheet named Capability. The User Input box at the top left records the selection and the settings, and the chart Capability - Diameter sits to its right. Because the Capability Analysis button starts with the histogram bars off, the chart plots probability density rather than bar counts:

Overall: the Normal curve from the mean and the overall standard deviation, drawn as a filled area (the part beyond a specification limit is filled in red)
Within: the Normal curve from the mean and the within standard deviation, drawn as a dashed line
LSL and USL: red vertical lines labelled LSL = 9.5 and USL = 10.5

The statistics tables to the right of the chart show:

Descriptive Statistics: Sample Size (N), Mean, StDev (Overall), StDev (Within), Minimum, Maximum
Specification Limits: the LSL and USL you entered
Process Capability: Cp, Cpl, Cpu, Cpk (from the within standard deviation) and Pp, Ppl, Ppu, Ppk (from the overall standard deviation). For this data Cpk is about 1.06 and Ppk is about 1.53
PPM (Defects Per Million): the Observed, Within and Overall parts per million below the LSL, above the USL, and in total
Goodness of Fit: the method (Anderson-Darling, a normality test) with its test statistic and p-value
Distribution: the distribution the curves use, which is Normal unless you pick another Fit Type on the Display Options tab

More Examples¶

Ready for more? See these variations:

Multiple Data Columns: Assess capability for multiple measurements with different spec limits
GroupBy: Compare capability across machines or shifts