AFALPHA procedure

Generates alpha designs (R.W. Payne).


Option

PRINT = string
Whether to print the design (design); default * i.e. no printing


Parameters

GENERATOR = matrices
generating array (of size number-of-plots-per-block by number-of-reps)

LEVELS = scalars or variates
Defines the levels of each treatment factor; if this is omitted, the levels of the TREATMENT factor are used, if available, otherwise LEVELS is determined from the generating array on the assumption that the blocks are to be of equal size

SEED = scalar
Seed to be used to randomize the design, if required

TREATMENTS = factors
Specifies the treatment factor for each design

REPLICATES = factors
Specifies the replicate factor

BLOCKS = factors
Specifies the block factor

UNITS = factors
Specifies the factor to index the units within each block


Description

Alpha designs are a very flexible class of resolvable incomplete block designs. A resolvable design is one in which each block contains only a selection of the treatments, but the blocks can be grouped together into subsets in which each treatment is replicated once. The groupings of blocks thus form replicates, and the block structure of the design is

Replicates / Blocks / Units

Such designs are particularly useful when there are many treatments to examine and the variability of the units is such that the block size needs to be kept small. Alpha designs were thus devised originally for the analysis of plant breeding trials (Patterson & Williams 1976), where many varieties may need to be evaluated in a single trial, and have the advantage that they can provide effective designs for any number of treatments.

   The construction of an alpha design requires a k × r array of integers between 0 and s-1, where r is the number of replicates, and s is the number of blocks per replicate. If the number of treatments, v, is a multiple of the number of blocks per replicate, k will be the number of units in each block, and v will be given by s × k. Otherwise, the design will have some blocks of size k and some of size k-1, and v will lie between s × (k-1) and s x k. Clearly, the properties of the design that is formed will be very dependent on the choice of array. Patterson, Williams and Hunter (1978) present 11 basic arrays to generate designs with up to 100 treatments and 2, 3 or 4 replicates when k is greater than 3 and s is greater than or equal to k; these arrays are reproduced in John (1987). Williams (1975) presents arrays for any sensible values of s and k with up to 100 treatments and 2 to 4 replicates.

   Procedure AFALPHA generates the treatment, replicate, block and unit factors for an alpha design. The design can be printed by setting option PRINT=design, and the factors can be saved using the parameters TREATMENTS, REPLICATES, BLOCKS and UNITS. The generating array for the design must be specified as a k × r matrix using the GENERATOR parameter, and the number of levels of the treatment factor can be defined by the LEVELS parameter. If LEVELS is omitted, AFALPHA will see whether the TREATMENTS parameter has been set to a factor whose levels have already been defined; if not, AFALPHA will set LEVELS to the scalar value v = s × k. By default the design is unrandomized, but randomization can be requested by setting the SEED parameter.


Option: PRINT.

Parameters: GENERATOR, LEVELS, SEED, TREATMENTS, REPLICATES, BLOCKS, UNITS.


Method

Each column of the generating array is used to form s-1 further columns by successively adding 1 modulo s. Next, s is added to row 2 of every column, 2s to row 3, and so on. Each resulting column then gives one of the blocks of the design, and the replicates are formed by the sets of columns that were all generated from the same initial column. If the design needs to have blocks of unequal sizes, procedure SUBSET is used to omit the necessary plots to form the smaller blocks.


References

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

Patterson, H.D., Williams E.R. & Hunter, E.A. (1978). Block designs for variety trials. Journal of Agricultural Science, Cambridge, 90, 395-400.

Williams, E.R. (1975). A new class of resolvable block designs. Ph.D. Thesis, University of Edinburgh.