Calculating Statistics

  1. On the home view, go to the Reports tab and select Statistics on the left pane.

  2. On the SPC Configuration window, enter the parameters to collect results stored in the controller (Number of Subgroups, Subgroup Size, and Frequency).

  3. Select the plus icon in the upper-right corner of the screen.
    The SPC Variable window shows the new variable.
    Configure the new SPC Variable.

  4. On the SPC Variable window, select Calculate.
    Data is collected from the historical results stored in the controller that are aligned with the criteria set in the SPC Configuration window and SPC Variable window. The values of interest are collected into subgroups of the size set in the SPC Configuration window. The collected tightening results on the controller are used for statistics calculations.

It is not possible to calculate statistics for results from deleted tightening programs or Virtual Stations, unless the results are still stored on the controller.

Only one user at a time can calculate statistics.

Statistics Formulas

Data in a subgroup is calculated as:

  • Average:

  • Range = R = max (Xi) - min (Xi), where i=1...n

  • Standard deviation for a subgroup:

  • Total overall of the standard deviation:

    Where N = total number of observations.

When specified number of subgroups to use for calculations has been collected, the SPC function starts calculations to determine the statistical stability.
Calculations are performed through the following steps: (m = number of subgroups)

  • Calculation of Average of Average values:

  • Calculation of Average of Range:

  • Calculation of Average of Standard deviation:

  • Calculation of the process capabilities Cp and Cpk with respect to Sigma:

    Where:

    • is the overall arithmetic average of the observed characteristic values.

    • UTL is the Upper Tolerance Limit.

    • LTL is the Lower Tolerance Limit.

    • The sigma estimation is the mean square standard deviation within the subgroups:

  • Calculation of the process capabilities Cp and Cpk with respect to Range:

    Where for a normal distribution su = sl = 3.

Calculations for control limits if, in the SPC Configuration window, the Auto calculate switch is set to Yes:

  • X-bar R (Mean)

  • X-bar R (Range)

  • X-bar S (Mean)

  • X-bar S (Sigma)

A2, A3, D3, D4, B3, and B4 are constants (for more information, see the "SPC Calculation Constants" paragraph).

SPC Calculations Constants

Subgroup Size

Divisors for estimate of standard dev.

Factors for Control Limits

Xbar R

Xbar S

A2

D3

D4

A3

B3

B4

n

d2

2

1.128

1.880

-

3.267

2.659

-

3.267

3

1.693

1.023

-

2.574

1.954

-

2.568

4

2.059

0.729

-

2.282

1.628

-

2.266

5

2.326

0.577

-

2.114

1.427

-

2.089

6

2.534

0.483

-

2.004

1.287

0.030

1.970

7

2.704

0.419

0.076

1.924

1.182

0.118

1.882

8

2.847

0.373

0.136

1.864

1.099

0.185

1.815

9

2.970

0.337

0.184

1.816

1.032

0.239

1.761

10

3.078

0.308

0.223

1.777

0.975

0.284

1.716

11

3.173

0.285

0.256

1.744

0.927

0.321

1.679

12

3.258

0.266

0.283

1.717

0.886

0.354

1.646

13

3.336

0.249

0.307

1.693

0.850

0.382

1.618

14

3.407

0.235

0.328

1.672

0.817

0.406

1.594

15

3.472

0.223

0.347

1.653

0.789

0.428

1.572

16

3.532

0.212

0.363

1.637

0.763

0.448

1.552

17

3.588

0.203

0.378

1.622

0.739

0.466

1.534

18

3.640

0.194

0.391

1.608

0.718

0.482

1.518

19

3.689

0.187

0.403

1.597

0.698

0.497

1.503

20

3.735

0.180

0.415

1.585

0.680

0.510

1.490

21

3.778

0.173

0.425

1.575

0.663

0.523

1.477

22

3.819

0.167

0.434

1.566

0.647

0.534

1.466

23

3.858

0.162

0.443

1.557

0.633

0.545

1.455

24

3.895

0.157

0.451

1.548

0.619

0.555

1.445

25

3.931

0.153

0.459

1.541

0.606

0.565

1.435