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In the statistical theory of the design of experiments, blocking is the arranging of experimental units in groups (blocks) that are similar to one another. Typically, a blocking factor is a source of variability that is not of primary interest to the experimenter. An example of a blocking factor might be the sex of a patient; by blocking on sex, this source of variability is controlled for, thus leading to greater accuracy.

Blocking to "remove" the effect of nuisance factors

For randomized block designs, there is one factor or variable that is of primary interest. However, there are also several other nuisance factors.

Nuisance factors are those that may affect the measured result, but are not of primary interest. For example, in applying a treatment, nuisance factors might be the specific operator who prepared the treatment, the time of day the experiment was run, and the room temperature. All experiments have nuisance factors. The experimenter will typically need to spend some time deciding which nuisance factors are important enough to keep track of or control, if possible, during the experiment.

Blocking used for nuisance factors that can be controlled

When we can control nuisance factors, an important technique known as blocking can be used to reduce or eliminate the contribution to experimental error contributed by nuisance factors. The basic concept is to create homogeneous blocks in which the nuisance factors are held constant and the factor of interest is allowed to vary. Within blocks, it is possible to assess the effect of different levels of the factor of interest without having to worry about variations due to changes of the block factors, which are accounted for in the analysis.

Definition of blocking factors

A nuisance factor is used as a blocking factor if every level of the primary factor occurs the same number of times with each level of the nuisance factor. The analysis of the experiment will focus on the effect of varying levels of the primary factor within each block of the experiment.

Block for a few of the most important nuisance factors

The general rule is:

"Block what you can; randomize what you cannot."

Blocking is used to remove the effects of a few of the most important nuisance variables. Randomization is then used to reduce the contaminating effects of the remaining nuisance variables.

Table of randomized block designs

One useful way to look at a randomized block experiment is to consider it as a collection of completely randomized experiments, each run within one of the blocks of the total experiment.

Randomized Block Designs (RBD)
Name of Design Number of Factors k Number of Runs n
2-factor RBD 2 L1 * L2
3-factor RBD 3 L1 * L2 * L3
4-factor RBD 4 L1 * L2 * L3 * L4
\vdots \vdots \vdots
k-factor RBD k L1 * L2 * \cdots * Lk

with

L1 = number of levels (settings) of factor 1
L2 = number of levels (settings) of factor 2
L3 = number of levels (settings) of factor 3
L4 = number of levels (settings) of factor 4
\vdots
Lk = number of levels (settings) of factor k

Example of a Randomized Block Design

Suppose engineers at a semiconductor manufacturing facility want to test whether different wafer implant material dosages have a significant effect on resistivity measurements after a diffusion process taking place in a furnace. They have four different dosages they want to try and enough experimental wafers from the same lot to run three wafers at each of the dosages.

The nuisance factor they are concerned with is "furnace run" since it is known that each furnace run differs from the last and impacts many process parameters.

An ideal way to run this experiment would be to run all the 4x3=12 wafers in the same furnace run. That would eliminate the nuisance furnace factor completely. However, regular production wafers have furnace priority, and only a few experimental wafers are allowed into any furnace run at the same time.

A non-blocked way to run this experiment would be to run each of the twelve experimental wafers, in random order, one per furnace run. That would increase the experimental error of each resistivity measurement by the run-to-run furnace variability and make it more difficult to study the effects of the different dosages. The blocked way to run this experiment, assuming you can convince manufacturing to let you put four experimental wafers in a furnace run, would be to put four wafers with different dosages in each of three furnace runs. The only randomization would be choosing which of the three wafers with dosage 1 would go into furnace run 1, and similarly for the wafers with dosages 2, 3 and 4.

Description of the experiment

Let X1 be dosage "level" and X2 be the blocking factor furnace run. Then the experiment can be described as follows:

k = 2 factors (1 primary factor X1 and 1 blocking factor X2)
L1 = 4 levels of factor X1
L2 = 3 levels of factor X2
n = 1 replication per cell
N = L1 * L2 = 4 * 3 = 12 runs

Before randomization, the design trials look like:

X1 X2
1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3
4 1
4 2
4 3

Matrix Representation

An alternate way of summarizing the design trials would be to use a 4x3 matrix whose 4 rows are the levels of the treatment X1 and whose columns are the 3 levels of the blocking variable X2. The cells in the matrix have indices that match the X1, X2 combinations above.

By extension, note that the trials for any K-factor randomized block design are simply the cell indices of a k dimensional matrix.

Model for a Randomized Block Design

The model for a randomized block design with one nuisance variable is


Y_{ij} = \mu + T_i + B_j + \mathrm{random\ error}

where

Yij is any observation for which X1 = i and X2 = j
X1 is the primary factor
X2 is the blocking factor
μ is the general location parameter (i.e., the mean)
Ti is the effect for being in treatment i (of factor X1)
Bj is the effect for being in block j (of factor X2)

Estimates for a Randomized Block Design

Estimate for μ : \overline{Y} = the average of all the data
Estimate for Ti : \overline{Y}_i - \overline{Y} with \overline{Y}_i = average of all Y for which X1 = i.
Estimate for Bj : \overline{Y}_j - \overline{Y} with \overline{Y}_j = average of all Y for which X2 = j.

Generalizations of randomized block designs

See also

External links

References

  • Addelman, Sidney (Oct. 1969). The Generalized Randomized Block Design. The American Statistician 23 (4): 35–36.
  • Addelman, Sidney (Sep. 1970). Variability of Treatments and Experimental Units in the Design and Analysis of Experiments. Journal of the American Statistical Association 65 (331): 1095–1108.
  • Caliński, Tadeusz and Kageyama, Sanpei (2000). Block designs: A Randomization approach, Volume I: Analysis, New York: Springer-Verlag.
  • Caliński, Tadeusz and Kageyama, Sanpei (2003). Block designs: A Randomization approach, Volume II: Design, New York: Springer-Verlag.
  • Gates, Charles E. (Nov. 1995). What Really Is Experimental Error in Block Designs?. The American Statistician 49 (4): 362–363.
  • Kempthorne, Oscar (1979). The Design and Analysis of Experiments, Corrected reprint of (1952) Wiley, Robert E. Krieger.
  • Hinkelmann, Klaus and Kempthorne, Oscar (2008). Design and Analysis of Experiments, Second, Wiley.
    • Hinkelmann, Klaus and Kempthorne, Oscar (2008). Design and Analysis of Experiments, Volume I: Introduction to Experimental Design, Second, Wiley.
    • Hinkelmann, Klaus and Kempthorne, Oscar (2005). Design and Analysis of Experiments, Volume 2: Advanced Experimental Design, First, Wiley.
  • Lentner, Marvin (1993). "The Generalized RCB Design (Chapter 6.13)" Experimental design and analysis, Second, 225–226, P.O. Box 884, Blacksburg, VA 24063: Valley Book Company.
  • Raghavarao, Damaraju (1988). Constructions and Combinatorial Problems in Design of Experiments, corrected reprint of the 1971 Wiley, New York: Dover.
  • Raghavarao, Damaraju and Padgett, L.V. (2005). Block Designs: Analysis, Combinatorics and Applications, World Scientific.
  • Shah, Kirti R. and Sinha, Bikas K. (1989). Theory of Optimal Designs, 171+viii, Springer-Verlag.
  • Street, Anne Penfold and Street, Deborah J. (1987). Combinatorics of Experimental Design, 400+xiv, Oxford U. P. [Clarendon].
  • Wilk, M. B. (June 1955). The Randomization Analysis of a Generalized Randomized Block Design. Biometrika 42 (1–2): 70–79.
  • Zyskind, George (Dec. 1963). Some Consequences of randomization in a Generalization of the Balanced Incomplete Block Design. The Annals of Mathematical Statistics 34 (4): 1569–1581.




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