Life data models in Minitab have these two restrictions:
- The model must be full rank. A full rank model includes enough data to estimate all the terms in your model. Missing data, insufficient data, or high collinearity may cause issues with full rank. Suppose you have a two-factor crossed model with one empty cell. You can then fit the model with terms A B, but not A B A*B. Minitab will alert you if the model is not full rank. Usually, eliminating some of the high order interactions in your model (assuming, of course, that they are not important) will solve your problem.
- The model must be hierarchical. In a hierarchical model, if an interaction term is included, all lower order interactions and main effects that compose the interaction term must be in the model.