The alias structure describes the confounding pattern that occurs in a design. Terms that are confounded are also said to be aliased.
Full aliasing occurs when the design does not include all of the combinations of factor levels. For example, if factor A is confounded with the 3-way interaction BCD, then the estimated effect for A is the sum of the effect of A and the effect of BCD. You cannot determine whether a significant effect is because of A, because of BCD, or because of a combination of both. If you try to include fully aliased terms in the same model, Minitab removes the fully aliased terms that are listed later in the terms list.
Partial aliasing is common in screening designs. In this case, mathematical estimates of the terms exist even though they represent combinations of different terms. Partial aliasing is less problematic in the screening design case because you expect that a small number of the terms are useful in the model. However, you should still be cautious when you interpret terms in a model with partial aliasing.
Use the alias structure to examine the aliased terms. For example, a study to evaluate the power of an ultrasonic cleanser has 7 factors:
- Train time (seconds): time duration for the amplitude modulation
- Degas time (seconds): time off for bubble release between modulations
- Burst time (milliseconds): time for high energy bubble bursts
- Quiet time (milliseconds): off time for the cleanser
- Center (kilohertz): center frequency away from 40 kilohertz
- Bandwidth (kilohertz): difference of the sweep from the center frequency
- Sweep time (seconds): time to complete 1 sweep
Screening design model: Power versus Train, Burst, Quiet, Center, Sweep
Alias Structure (up to order 2)
I + 0.51 AA + 0.51 BB + 0.64 CC + 0.77 DD + 0.64 EE + 0.51 FF + 0.77 AB
- 0.90 AC + 0.26 AD + 0.90 AE - 0.77 AF - 0.64 BC + 0.77 BD + 0.64 BE - 0.77 BF
+ 0.51 BG - 0.64 CD + 0.64 CF - 0.13 CG + 0.90 DE - 0.26 DF - 0.64 EF - 0.13 EG
+ 0.51 FG
GG + 0.38 AA + 0.38 BB + 0.22 CC + 0.06 DD + 0.22 EE + 0.38 FF - 0.94 AB
+ 1.09 AC - 0.31 AD - 1.09 AE + 0.94 AF + 0.78 BC - 0.94 BD - 0.78 BE + 0.94 BF
- 0.62 BG + 0.78 CD - 0.78 CF + 0.16 CG - 1.09 DE + 0.31 DF + 0.78 EF + 0.16 EG
- 0.62 FG
CE - 0.23 AA - 0.23 BB - 0.04 CC + 0.16 DD - 0.04 EE - 0.23 FF + 0.16 AB
- 0.35 AC - 0.61 AD + 0.35 AE - 0.16 AF + 0.04 BC + 0.16 BD - 0.04 BE - 0.16 BF
+ 0.77 BG + 0.04 CD - 0.04 CF - 0.19 CG + 0.35 DE + 0.61 DF + 0.04 EF - 0.19 EG
+ 0.77 FG
The pareto chart shows that 2 of the main effects, 1 interaction term, and 1 quadratic effect are significant at the 0.05 level. The alias structure shows that the constant term, the quadratic term, and the interaction term are partially aliased with other effects. In the screening case, you usually assume that the strongest effects associate with the changes in the response. If the experiment does not satisfactorily identify the most important terms, the partial aliasing is 1 possible cause. If the partial aliasing is the cause, then you can collect more data to learn about more of the effects independently of other effects.