Predictive and Specialized Modeling > Process Screening > Statistical Details for the Process Screening Platform
Publication date: 08/13/2020

Statistical Details for the Process Screening Platform

Scaling Factors for Using Medians to Estimate Sigma

When you select Use Medians instead of Means, sigma is estimated using a scaled median range or median standard deviation. The table below gives the scaling factors, which were obtained using Monte Carlo simulation.

For subgroups of size n drawn from a normal distribution, the following are true:

The theoretical median of the ranges is approximately d2_Median*σ, where d2_Median is the value corresponding to n.

The theoretical median of the standard deviations is approximately c4_Median*σ, where c4_Median is the value corresponding to n.

Table 22.2 Scaling Constants for Median Range and Median Standard Deviation

n

d2_Median

c4_Median

2

0.953

0.675

3

1.588

0.833

4

1.978

0.888

5

2.257

0.917

6

2.471

0.933

7

2.646

0.944

8

2.792

0.952

9

2.915

0.959

10

3.024

0.963

11

3.118

0.967

12

3.208

0.969

13

3.286

0.972

14

3.357

0.975

15

3.422

0.976

16

3.483

0.978

17

3.539

0.979

18

3.590

0.980

19

3.640

0.981

20

3.685

0.982

21

3.731

0.983

22

3.770

0.984

23

3.811

0.984

24

3.846

0.985

25

3.883

0.986

Want more information? Have questions? Get answers in the JMP User Community (community.jmp.com).
.