You can enter numeric, text, or date/time factor levels. Minitab needs to assign one factor level to be the reference level, meaning that the estimated coefficients are interpreted relative to this level.
Probit analysis creates a set of design variables for the factor in the model. If there are k levels, there will be k-1 design variables and the reference level will be coded with all 0's. 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 |
1 2 3 4 |
A1 0 1 0 0 |
A2 0 0 1 0 |
A3 0 0 0 1 |
reference |
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.