Factor variables and reference levels
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You can enter numeric, text, or date/time factor levels. Minitab assigns one factor level to be the reference level, meaning that the estimated coefficients are interpreted relative to this level.

Regression with Life Data creates a set of design variables for each factor in the model. If there are k levels, there will be k - 1 design variables and the reference level will be coded as 0. Here are two examples of the default coding scheme:

 

Factor A with 4 levels

 

Factor B with 3 levels

 

(1 2 3 4)

 

(High Low Medium)

reference
level is 1

 

1

2

3

4

A1

0

1

0

0

A2

0

0

1

0

A3

0

0

0

1

reference
level is High

 

High

Low

Medium

B1

0

1

0

B2

0

0

1

 

By default, Minitab designates the lowest numeric, date/time, or text value as the reference factor level. If you like, you can change this reference value in the Options subdialog box.