Refinement
After fitting a model in Adams Insight, it is important to evaluate the quality (or "Goodness") of the fit. If the fit does not meet your criteria, you can refine it using Adams Insight.
You can perform a
manual or
automatic refinement.
Manual refinement
To manually refine the fit of your model:
1. From the Tools menu, point to Refine Model Manually, and then select one of the following:
■Remove Outliers: Select
Outliers to remove from your experiment.
■Remove Terms: Select terms to remove from your experiment.
■Transform Response: Specify the type of transformation for your response.
■Change Order: Specify the model order for your experiment.
2. Enter the information as described in the dialog boxes:
3. Select OK.
Automatic (stepwise) refinement
Adams Insight uses stepwise refinement to automatically refine regression polynomials. Stepwise refinement repeatedly adds and removes terms in the regression equation, trying to find the smallest set of significant terms. Stepwise refinement is not guaranteed to find the smallest set, but usually gives good results.
The stepwise refinement will not increase the order of the polynomial. For example, if you have a quadratic order regression, the stepwise refinement might include some or all of the quadratic terms, but will not add any cubic terms.
You can manually review and modify the terms the stepwise refinement has excluded using the
Manual refinement method described above.
The basic stepwise refinement process is:
1. Start with the constant term.
2. If possible, add a significant term or group of terms to the polynomial.
3. If possible, remove an insignificant term or group or terms from the polynomial.
4. Repeat steps 2 and 3 until no other terms can be added or removed, or until a term combination is repeated.
Adams Insight repeats the stepwise refinement on each regression in the specified model.
Depending on the hierarchy option you choose, Adams Insight may add or remove individual terms, or only groups of terms. Hierarchy refers to the relationship between polynomial terms, and whether or not some terms should be added or removed before others.
At each step, the stepwise refinement evaluates the terms or sets of terms that could be added or removed based on hierarchy. It evaluates them using your choice of F or P value, and your specified threshold value to add or remove terms. It first evaluates all possible terms or sets of terms to add. If the best term or set is more significant than your specified threshold, it will add the term or set. If not, no term is added. Then, it evaluates all possible terms or sets of terms to remove. If the worst term or set is less significant than your specified threshold, it will remove it. If not, no term is removed.
The refinement ends when Adams Insight cannot add or remove any more terms, or when a term combination is repeated.
Many of the term evaluations require a new computed fit. As the number of terms increases, not only does the number of evaluations increase, but the time to compute the fit increases as well. For large models, stepwise refinement may be quite slow, especially with no hierarchy.
To automatically refine the fit of your model:
1. From the Tools menu, point to Refine Model Automatically, and then select one of the following:
■Remove Terms: Automatically removes terms from your experiment.
■Remove Outliers: Automatically removes outliers from your experiment.
2. Enter the information as described in the dialog boxes:
3. Select Start (or OK) to begin the refinement.
Adams Insight displays the messages in the Monitor tab. If you want to stop the process before it is complete, select Stop or Close. If you stop the refinement, no results are saved.