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GroupBy

Use GroupBy to create separate Cpk/Histograms for each group in your data, making it easy to compare process capability across machines, shifts, or production lines.

Goal

Compare diameter measurement capability between Machine A and Machine B, both using the same specification limits (LSL = 9.5, USL = 10.5).

Sample Data

Download CpkHistogram_GroupBy.xlsx

Excel Protected View

When you open downloaded files, Excel displays a Protected View warning. You must click Enable Editing before you can use Quantum XL with the file.

Excel Protected View warning

Alternatively, you can copy the sample data from the table below and paste it directly into a new Excel workbook.

Diameter Machine
9.82 Machine A
10.05 Machine A
9.97 Machine A
10.12 Machine A
9.91 Machine A
10.25 Machine B
9.78 Machine B
10.35 Machine B
9.65 Machine B
10.18 Machine B

Each row represents one diameter measurement tagged with the machine that produced it. Machine A has 5 observations centered tightly around 10.0, while Machine B has 5 observations with more spread.

Steps

  1. Launch the analysis

    From the Excel ribbon, select QXL Stat Tools → Analysis Tools → Create Histogram.

  2. Select your data

    Select cells A1:B11 (the header row plus all 10 data rows).

  3. Configure the analysis

    In the Cpk/Histogram dialog:

    • Select the GroupBy radio button (instead of Excel)
    • Data Columns: "Diameter" should be checked, "Machine" should be unchecked
    • GroupBy: Move "Machine" to the GroupBy Order list
    • Specification Limits for Diameter: LSL = 9.5, USL = 10.5

    Click Finish to generate the charts.

How GroupBy Works

When you add a GroupBy column, Quantum XL creates a separate Cpk/Histogram for every unique value in that column. In this example, the "Machine" column has two unique values — "Machine A" and "Machine B" — so we get two charts, each with its own capability statistics.

Result

Quantum XL creates two Cpk/Histograms, one for each machine:

Machine A — Diameter measurements are tightly centered around 10.0 with a narrow spread, resulting in a higher Cpk value.

Machine B — Diameter measurements have more variability (spread from 9.65 to 10.35), resulting in a lower Cpk value despite using the same specification limits.

Comparing Cpk values between machines reveals which machine has better process capability. A machine with a Cpk below 1.0 may need process improvement.