Design of Experiments

  • Four Questions that Define Which DOE is Right for You

    Do you ever stare at the broad array of DOE choices and wonder where to start? Which design is going to provide you with the information needed to solve your problem? I’ve boiled this down to a few key questions. Each of them may trigger more in-depth conversation, but the answers are key to driving your design decisions.

    1. What is the purpose of your experiment? Typical purposes are screening, characterization, and optimization. The screening design will help identify main effects (it’s important to choose a design that will estimate main effects separately from two-factor interactions (2FI)). Characterization designs will estimate 2FI’s and give you the option to add center points to detect curvature. Optimization designs generally estimate non-linear, or quadratic effects. (See the blog “A Winning Strategy for Experimenters”.)

    2. Are your factors actually components in a formulation? This leads you to a mixture design. Consider this question – if you double all the components in the process, will the response be the same? If yes, then only mixture designs will properly account for the dependencies in the system. (Check out the Formulation Simplified textbook.)

    3. Do you have any Hard-to-Change factors? An example is temperature – it’s hard to randomly vary the temp setting higher and lower due to the time required to stabilize the process. If you were planning to sort your DOE runs manually to make it easier to run the experiment, then you likely have a hard-to-change factor. In this case, a split-plot design will give a more appropriate analysis.

    4. Are your factors all numeric, or all categoric, or some of each? Multilevel categoric designs work better with categoric factors that are set at more than 2 levels. A final option - Optimal designs are highly flexible and can usually meet your needs for all factor types and require only minimal runs.

    These questions, along with your budget for number of runs, will guide your decisions regarding what type of information is important to your business, and what type of factors you are using in the experiment. Conveniently, the Design Wizard in Design-Expert® software (pictured below) asks these questions, guiding you through the decision-making process, ultimately leading you to a recommended starting design.

    Click image above for larger (and sharper) version Click image above for larger (and sharper) version
    Click on the options in the wizard to read more about each choice.

    Give it a whirl – Happy Experimenting!

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  • A Winning Strategy for Experimenters!

    A winning business strategy lays out a path with small steps that allows for changes in direction along the way. Our “SCO” flowchart for experimenters is a prime example of such a template for success. Its tried-and-true* core is screening (“S”), characterization (“C”) and optimization (“O”). However, we added one last, but perhaps most important, step: Confirmation. Let’s dive into the Stat-Ease strategy for experimenters and find out what makes it work so well.

    Stat-Ease SCO Flowchart Stat-Ease SCO Flowchart

     

    Our starting point is the Screening design. Screening designs provide a broad, but shallow, search for previously unknown process factors. TIPdon’t bother screening factors that are already known to affect your responses! Newly discovered factors—the “vital few” carry forward into the next phase of experimentation, with the “trivial many” being cast aside. By using medium-resolution (Res IV) designs—color-coded yellow in the primary two-level factorial builder in Design-Expert® software (DX), you can screen for main effects even in the presence hidden interactions. If runs must be closely budgeted, take advantage of the unique Minimum-Run Screening designs in DX.

    Moving ahead to Characterization with the vital-few screened factors plus the big one(s) you set aside, the identification of two-factor interactions becomes the goal. This necessitates a high-resolution design (Res V or better)—the green ones in DX’s main builder. To save runs, consider a Minimum-Run Characterization design. Either way, be sure to add center points at this stage so you can check curvature. If curvature is NOT significant, then your mission is nearly complete—all that remains is Confirmation!

    If curvature does emerge as being significant and important, then move on to Optimization using response surface methods (RSM). The beauty of RSM is that, with the aid of DX and its modeling and graphics tool, you can see by contour and 3D maps where each response peaks. Also, via numerical tools, DX can pinpoint the setup of factors producing the most desirable outcome for multiple responses. Then it lays out a compelling visual of the sweet spot—the window where all specifications can be achieved.

    Last, but not least, comes Confirmation, during which you do a number of runs to be sure you can reproduce the good results. Use the special tool for confirmation that DX provides to be confident of this.

    In conclusion, DOE does not provide a single template that you can repeat over and over. You must apply a strategy, such as the one outlined here, that adapts at each stage of your journey to a new and improved process that saves money at an improved quality level. Why not go after it all!

    Learn more about the Stat-Ease strategy for experimenters by attending the Modern DOE for Process Optimization workshop or by reading the DOE Simplified textbook.

    *Strategy of experimentation: Break it into a series of smaller stages, Mark Anderson, StatsMadeEasy blog, 6/20/11.

  • Energize Two-Level Factorials - Add Center Points!

    Two-level factorial designs are highly effective for discovering active factors and interactions in a process, and are optimal for fitting linear models by simply comparing low vs high factor settings. Super-charge these classic designs by adding center points!
    (Read to the end for a bonus video clip!)

    Picture1 Cpts Continue reading

  • How To Save Costly Project Engineering Time With This Innovative Application of Response Surface Models (RSM)

    By Guest Author Nate Kaemingk

    Lead engineers are incredibly overworked and their time is disproportionately valuable compared to some more junior engineers. But when it comes to doing project cost estimations, we don't have much of a choice but to ask some of these high-value engineers for the estimates if we want a realistic level of accuracy. Plus, the unfortunate reality is that many project costs end up being too high to justify the screening business case Continue reading

  • Five Keys to Increase ROI for DOE On-Site Training

    By Shari Kraber

    A recent discussion with a client led to these questions—“How do we keep design of experiments (DOE) training “alive” so that long-term benefits can be seen? How do we ensure our employees will apply their new-found skills to positively impact the business?” In my 20+ years as a DOE consultant and trainer, I have seen many companies who invested in on-site training, only to have it die a quick death mere days Continue reading

  • Adding Intervals to Optimization Graphs

    Design-Expert® software provides powerful features to add confidence, prediction, or tolerance intervals to its graphical optimization plots. All users can benefit by seeing how this provides a more conservative ‘sweet spot’. However, this innovative enhancement is of particular value for those in the pharmaceutical industry who hope to satisfy the US FDA’s QbD (quality by design) requirements.

    Here are the definitions: Continue reading

  • Choosing the Best Design for Process Optimization

    Ever wonder what the difference is between the various response surface method (RSM) optimization design options? To help you choose the best design for your experiment, I’ve put together a list of things you should know about each of the three primary response surface designs—Central Composite, Box-Behnken, and Optimal.

    Central Composite Design (CCD)

  • Tips and Tricks for Navigating Design-Expert® Software

    We’ve designed Design-Expert® software to be flexible and user-friendly. For those of you who haven’t had a chance to fully explore its capabilities, here are some tips to help you navigate the software and find options that are useful for you:

    • The all-new Design Wizard asks you a series of questions, and then directs you to a starting design! You may want to modify things from here, but it’s a great starting Continue reading
  • Don’t Let R^2 Fool You

    By Pat Whitcomb

    Has a low R2 ever disappointed you during the analysis of your experimental results? Is this really the kiss of death? Is all lost? Let’s examine R2 as it relates to factorial design of experiments (DOE) and find out.

    R2 measures are calculated on the basis of the change in the response (Δy) relative to the total variation of the response (Δy + σ) Continue reading

  • DOE Simplified, 3rd Edition Now Available

    The third edition of DOE Simplified: Practical Tools for Effective Experimentation is now available. This comprehensive introductory text is geared towards readers with a minimal statistical background. In it, the authors take a fresh and lively approach to learning the fundamentals of experiment design and analysis. This edition includes a major revision of the software that accompanies the book (via trial download) and sets the stage for introducing experiment designs where the Continue reading

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