The Fit Two Level Screening platform has a carefully defined order of operations:
1. First, the main effect terms enter the model according to the absolute size of their contrast. All effects are orthogonalized to the effects preceding them in the model. This method assures that their order is the same as it would be in a forward stepwise regression. Ordering by main effects also helps in selecting preferred aliased terms later in the process.
2. After main effects, all second-order interactions enter the model, followed by third-order interactions, and so on. The second-order interactions cross with all earlier terms before bringing in a new term because B and C are both larger than D. The complete order of entry is AB, AC, BC, AD, BD, and then CD.
3. An effect that is an exact alias for an effect already in the model appears in the Alias column. Effects that are a linear combination of several previous effects do not appear. If there is partial aliasing (due to a lack of orthogonality), the effects involved are marked with an asterisk.
4. The process continues until n effects are obtained, where n is the number of rows in the data table, thus fully saturating the model. If complete saturation is not possible with the factors, JMP generates random orthogonalized effects to absorb the rest of the variation. They are labeled Null n where n is a number. For example, this situation occurs if there are exact replicate rows in the design.