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5. Process Improvement
5.5. Advanced topics
5.5.9. An EDA approach to experimental design

Block plot

Purpose The block plot answers the following two general questions:
  1. What are the important factors (including interactions)?
  2. What are the best settings for these important factors?
The basic (single) block plot is a multifactor EDA technique to determine if a factor is important and to ascertain if that importance is unconditional (robust) over all settings of all other factors in the system. In an experimental design context, the block plot is actually a sequence of block plots with one plot for each of the k factors.

Due to the ability of the block plot to determine whether a factor is important over all settings of all other factors, the block plot is also referred to as a DOE robustness plot.

Output The block plot provides specific information on
  1. Important factors (of the k factors and the \( \left( \begin{array}{c} k \\ 2 \end{array} \right) \) 2-factor interactions); and
  2. Best settings of the important factors.
Definition The block plot is a series of k basic block plots with each basic block plot for a main effect. Each basic block plot asks the question as to whether that particular factor is important:
  1. The first block plot asks the question: "Is factor X1 important?
  2. The second block plot asks the question: "Is factor X2 important?
  3. Continue for the remaining factors.
The i-th basic block plot, which targets factor i and asks whether factor Xi is important, is formed by:
  • Vertical Axis: Response

  • Horizontal Axis: All 2k-1 possible combinations of the (k-1) non-target factors (that is, "robustness" factors). For example, for the block plot focusing on factor X1 from a 23 full factorial experiment, the horizontal axis will consist of all 23-1 = 4 distinct combinations of factors X2 and X3. We create this robustness factors axis because we are interested in determining if X1 is important robustly. That is, we are interested in whether X1 is important not only in a general/summary kind of way, but also whether the importance of X is universally and consistently valid over each of the 23-1 = 4 combinations of factors X2 and X3. These four combinations are (X2, X3) = (+, +), (+, -), (-, +), and (-, -). The robustness factors on the horizontal axis change from one block plot to the next. For example, for the k = 3 factor case:

    1. the block plot targeting X1 will have robustness factors X2 and X3;
    2. the block plot targeting X2 will have robustness factors X1 and X3;
    3. the block plot targeting X3 will have robustness factors X1 and X2.

  • Plot Character: The setting (- or +) for the target factor Xi. Each point in a block plot has an associated setting for the target factor Xi. If Xi = "-", the corresponding plot point will be "-"; if Xi = "+", the corresponding plot point will be "+".
For a particular combination of robustness factor settings (horizontally), there will be two points plotted above it (vertically):
  1. one plot point for Xi = "-"; and
  2. the other plot point for Xi = "+".
In a block plot, these two plot points are surrounded by a box (a block) to focus the eye on the internal within-block differences as opposed to the distraction of the external block-to-block differences. Internal block differences reflect on the importance of the target factor (as desired). External block-to-block differences reflect on the importance of various robustness factors, which is not of primary interest.

Large within-block differences (that is, tall blocks) indicate a large local effect on the response which, since all robustness factors are fixed for a given block, can only be attributed to the target factor. This identifies an "important" target factor. Small within-block differences (small blocks) indicate that the target factor Xi is unimportant.

For a given block plot, the specific question of interest is thus

    Is the target factor Xi important? That is, as we move within a block from the target factor setting of "-" to the target factor setting of "+", does the response variable value change by a large amount?
The height of the block reflects the "local" (that is, for that particular combination of robustness factor settings) effect on the response due to a change in the target factor settings. The "localized" estimate for the target factor effect for Xi is in fact identical to the difference in the response between the target factor Xi at the "+" setting and at the "-" setting. Each block height of a robustness plot is thus a localized estimate of the target factor effect.

In summary, important factors will have both

  1. consistently large block heights; and
  2. consistent +/- sign arrangements
where the "consistency" is over all settings of robustness factors. Less important factors will have only one of these two properties. Unimportant factors will have neither property.
Plot for defective springs data Applying the ordered response plot to the defective springs data set yields the following plot.

Block plots for the defective springs data

How to interpret From the block plot, we are looking for the following:
  1. Important factors (including 2-factor interactions);
  2. Best settings for these factors.
We will discuss each of these in turn.

Important factors (including 2-factor interactions):

Look at each of the k block plots. Within a given block plot,

    Are the corresponding block heights consistently large as we scan across the within-plot robustness factor settings--yes/no; and are the within-block sign patterns (+ above -, or - above +) consistent across all robustness factors settings--yes/no?
To facilitate intercomparisons, all block plots have the same vertical axis scale. Across such block plots,
  1. Which plot has the consistently largest block heights, along with consistent arrangement of within-block +'s and -'s? This defines the "most important factor".

  2. Which plot has the consistently next-largest block heights, along with consistent arrangement of within-block +'s and -'s? This defines the "second most important factor".

  3. Continue for the remaining factors.
This scanning and comparing of the k block plots easily leads to the identification of the most important factors. This identification has the additional virtue over previous steps in that it is robust. For a given important factor, the consistency of block heights and sign arrangement across robustness factors gives additional credence to the robust importance of that factor. The factor is important (the change in the response will be large) irrespective of what settings the robustness factors have. Having such information is both important and comforting.

Important Special Case; Large but Inconsistent:

What happens if the block heights are large but not consistent? Suppose, for example, a 23 factorial experiment is being analyzed and the block plot focusing on factor X1 is being examined and interpreted so as to address the usual question of whether factor X1 is important.

Let us consider in some detail how such a block plot might appear. This X1 block plot will have 23-1 = 4 combinations of the robustness factors X2 and X3 along the horizontal axis in the following order:

    (X2, X3) = (+, +); (X2, X3) = (+, -); (X2, X3) = (-, +); (X2, X3) = (-, -).
If the block heights are consistently large (with "+" above "-" in each block) over the four combinations of settings for X2 and X3, as in

    (X2, X3) block height (= local X1 effect)
    (+, +) 30
    (+, -) 29
    (-, +) 29
    (-, -) 31

then from binomial considerations there is one chance in 24-1 = 1/8 \( \approx \) 12.5 % of the the four local X1 effects having the same sign (i.e., all positive or all negative). The usual statistical cutoff of 5 % has not been achieved here, but the 12.5 % is suggestive. Further, the consistency of the four X1 effects (all near 30) is evidence of a robustness of the X effect over the settings of the other two factors. In summary, the above suggests:

  1. Factor 1 is probably important (the issue of how large the effect has to be in order to be considered important will be discussed in more detail in a later section); and
  2. The estimated factor 1 effect is about 30 units.
On the other hand, suppose the 4 block heights for factor 1 vary in the following cyclic way:

    (X2, X3) block height (= local X1 effect)
    (+, +) 30
    (+, -) 20
    (-, +) 30
    (-, -) 20

then how is this to be interpreted?

The key here to such interpretation is that the block plot is telling us that the estimated X1 effect is in fact at least 20 units, but not consistent. The effect is changing, but it is changing in a structured way. The "trick" is to scan the X2 and X3 settings and deduce what that substructure is. Doing so from the above table, we see that the estimated X1 effect is 30

  • for point 1 (X2, X3) = (+, +) and
  • for point 3 (X2, X3) = (-, +)
and then the estimated X1 effect drops 10 units to 20
  • for point 2 (X2, X3) = (+, -) and
  • for point 4 (X2, X3) = (-, -)
We thus deduce that the estimated X1 effect is
  1. 30 whenever X3 = "+"
  2. 20 whenever X3 = "-"
When the factor X1 effect is not consistent, but in fact changes depending on the setting of factor X3, then definitionally that is said to be an "X1*X3 interaction". That is precisely the case here, and so our conclusions would be:
  1. factor X1 is probably important;
  2. the estimated factor X1 effect is 25 (the average of 30, 20, 30, and 20);
  3. the X1*X3 interaction is probably important;
  4. the estimated X1*X3 interaction is about 10 (the change in the factor X1 effect as X3 changes = 30 - 20 = 10);
  5. hence the X1*X3 interaction is less important than the X1 effect.
Note that we are using the term important in a qualitative sense here. More precise determinations of importance in terms of statistical or engineering significance are discussed in later sections.

The block plot gives us the structure and the detail to allow such conclusions to be drawn and to be understood. It is a valuable adjunct to the previous analysis steps.

Best settings:

After identifying important factors, it is also of use to determine the best settings for these factors. As usual, best settings are determined for main effects only (since main effects are all that the engineer can control). Best settings for interactions are not done because the engineer has no direct way of controlling them.

In the block plot context, this determination of best factor settings is done simply by noting which factor setting (+ or -) within each block is closest to that which the engineer is ultimately trying to achieve. In the defective springs case, since the response variable is percent acceptable springs, we are clearly trying to maximize (as opposed to minimize, or hit a target) the response and the ideal optimum point is 100 %. Given this, we would look at the block plot of a given important factor and note within each block which factor setting (+ or -) yields a data value closest to 100 % and then select that setting as the best for that factor.

From the defective springs block plots, we would thus conclude that

  1. the best setting for factor 1 is +;
  2. the best setting for factor 2 is -;
  3. the best setting for factor 3 cannot be easily determined.
Conclusions for the defective springs data In summary, applying the block plot to the defective springs data set results in the following conclusions:
  1. Unranked list of important factors (including interactions):

    • X1 is important;
    • X2 is important;
    • X1*X3 is important.

  2. Best Settings:

      (X1, X2, X3) = (+, -, ?) = (+1, -1, ?)
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