## Introduction

An incomplete block design (IBD) of size (*v,k,r*) is an arrangement of
*v* treatments set out in blocks of size *k* (<*v*) such that
each treatment is replicated *r* times. We confine to *binary* IBDs,
i.e. IBDs in which no treatment occurs more than once in a block. An IBD is
said to be a *balanced* IBD, or BIBD, if every pair of treatments occurs
in exactly blocks. IBDs in Examples 2, 4 and 9 are examples of BIBDs.
An IBD is said to be *t*-resolvable if its blocks can be divided into
subsets and each subset is an IBD of size (*v,k,t*). IBDs in Examples 1,
6, 7, 8 and 9 are examples of 1-resolvable IBDs or resolvable IBDs. IBDs in
Examples 2 and 10 are examples of 2-resolvable IBDs. Some recommended references
on IBDs are John (1980), John & Williams (1995) and Raghavarao & Padgett (2005).
See Nguyen & Blagoeva (2010) for a quick reference on this subject.

IBD is a Gendex program for constructing optimal or near-optimal IBDs
(resolvable and non-resolvable). The optimality criterion and the algorithm
used in IBD is discussed in Nguyen (1993, 1994). Designs produced by IBD are
comparable to α-designs of Patterson & Williams (1976) and
generalized cyclic designs of Hall & Jarrett (1981) in terms of the
efficiency factor *E* of the design. These designs, in turn, can be used
as column components of row-column designs (Nguyen & Williams 1993; Nguyen
1997).

## Using IBD

Let's assume all Gendex class files are in the directory c:\gendex and
suppose you want to construct a resolvable IBD of size (*v,k,r*)=(6,2,4)
(Example 1). At the working directory, type the following command at the
Command Prompt (case is important):

java -cp c:\gendex IBD

The IBD GUI will pop up. Enter the values 6, 2 and 4 in the
(*v,k,r*) combination fields, the IBD GUI will become:

Now click **START**, since the combination
(*v,k,r*)=(6,2,4) allows resolvable IBD, the following question will pop up:

Click **Yes**, IBD will start running and after try 1, the plan of the
constructed design pops up in the IBD output window (as the ratio E/U reaches 1)
and IBD stops:

**START**button has changed to

**RESET**button. If you click this

**RESET**button, the output will disappear and you can use IBD for a new design problem. Note that the default random seed is the one obtained from the system clock and the default number of tries is 1000. You can change these default values if you wish to.

## Output

The result of the best try is displayed in the IBD output window and is also saved in
the file IBD.htm in the working directory. This file can be read by a
browser such as IE or *Google Chrome*. Information for this try includes:

- Try number;
- The number of iterations;
*E*, the efficiency factor of this design.- The ratio
*E/U*where*U*is the upper bound of an IBD.*U*=*U*for non-resolvable IBDs and_{J}*U*=min(*U*,_{J}*U*) for resolvable IBDs._{WP}*U*is the bound of Williams & Patterson (1977) good for any resolvable IBDs with_{WP}*v≥b*.*U*is the bound of Jarrett (1989) good for any regular graph design (RGD). RGDs are IBDs with concurrences differing by at most 1. IBDs in Examples 1-6 and 8-10 are examples of RGDs. The program automatically stops if this ratio reaches 1._{J} - The distribution of the concurrences of this design;
- The design plan and the associated random seed;
- The time in seconds IBD used to construct this design.

An additional output of a IBD session is the file form.htm. This file contains the randomized plan of the constructed design.

## Examples

- A resolvable IBD of size (
*v,k,r*)=(6,2,4) (http://designcomputing.net/gendex/ibd/b1.html). - A 2-resolvable BIBD of size (
*v,k,r*)=(6,4,10). (http://designcomputing.net/gendex/ibd/b2.html). - An IBD of size (
*v,k,r*)=(14,5,10) (http://designcomputing.net/gendex/ibd/b3.html). - A BIBD of size (
*v,k,r*)=(15,5,14) (http://designcomputing.net/gendex/ibd/b4.html). - An IBD of size (
*v,k,r*)=(60,9,3) (http://designcomputing.net/gendex/ibd/b5.html). - A resolvable IBD of size (
*v,k,r*)=(30,5,4) (http://designcomputing.net/gendex/ibd/b6.html). - A resolvable IBD of size (
*v,k,r*)=(36,6,4) (http://designcomputing.net/gendex/ibd/b7.html). - A resolvable IBD of size (
*v,k,r*)=(98,7,2) (http://designcomputing.net/gendex/ibd/b8.html). - A resolvable BIBD of size (
*v,k,r*)=(15,3,7) (http://designcomputing.net/gendex/ibd/b9.html). - A 2-resolvable IBD of size (
*v,k,r*)=(21,6,10) (http://designcomputing.net/gendex/ibd/b10.html).

**Notes**:

- Example 2: See Box, Hunter & Hunter (1987) p. 271 for an alternative solution to the BIBD.
- Example 4: This BIBD has been listed as unsolved in Raghavarao (1971) p. 95.
- Example 9: See Street & Street (1987) p. 164 for an alternative solution to this BIBD, also known as Kirkman's school girl problem.
- Example 10: See Clatworthy (1973) plan
**T69**for an alternative solution. Clearly,**T69**is a very poor design as it does not makes sense for certain pairs of treatments to appear together in five blocks while certain pairs never appear together in any of the blocks.

## References

Box, G.E.P, Hunter, W.G. & Hunter, J.S. (1978) *Statistics for
experimenters*. New York: John Wiley.

Clatworthy, W.H. (1973) *Tables of two-associates-class partially balanced
designs.* Appl. Math. Ser. 63. National Bureau of Standards, Washington.

Hall, W.B. & Jarrett, R.G. (1981) Non-resolvable incomplete block designs
with few replications. *Biometrika* **68**, 617-627.

Jarrett, R.G. (1989) A review of bounds for the efficiency factor of block
designs. *Austral. J. Statist.* **31**, 118-129.

John, J.A. & Williams E.R. (1987) *Cyclic designs and computer-generated
designs*. New York: Chapman & Hall.

John, P.W.M. (1980) *Incomplete block designs.* New York: Marcel Decker,
Inc.

Nguyen, N-K. (1993) An algorithm for constructing optimal resolvable block
designs. *Commun. Statist. B* **22**, 911-923.

Nguyen, N-K. (1994) Construction of optimal incomplete block designs by
computer. *Technometrics* **36**, 300-307.

Nguyen, N-K. and Williams, E.R. (1993) An algorithm for constructing optimal
resolvable row-column designs. *Austral. J. Statist.* **35**,
363-370.

Nguyen, N-K. (1997) Construction of optimal row-column designs by computer.
*Computing Science & Statistics* **28**, 471-475.

Nguyen, N-K. & Blagoeva K.L. (2010) Incomplete block designs, in
*Encyclopedia of Statistical Science*, Edited by L. Miodrag. Springer, 653-655.

Patterson, H.D. & Williams, E.R. (1976) A class of resolvable incomplete
block designs. *Biometrika* **63**, 83-92.

Raghavarao, D. & Padgett L.V. (2005). *Block Designs: Analysis, Combinatorics and Applications*. World Scientific.

Williams & Patterson (1977) Upper bound for efficiency factors in block
designs. *Austral. J. Statist.* **19**, 194-201.

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