THESIS

1.2.2 Cluster analysis
I. Ratio scale traits (quantitative traits)
The 69 accessions belonging to Solanum spp. was analysed based on
the 13 traits selected using the PROC CLUSTER statement. This procedure searched
throughout the data for accessions that were similar enough to be considered as part of
a common cluster (Cababasay, 1996).
Using the standardized data, the Unweighted Pair Group of
Matheretic Average (UPGMA) was chosen to stratify the 69 accessions. This could be
attributed to the fact that average linkage method (UPGMA) can be used with any
resemblance coefficients compared with Ward’s method. Moreover, the similarity
between pairs of clusters can be determined in a manner less extreme compare with
single or complete linkage (Romesburg, 1984). This was done by taking the average
distance between all pairwise combinations of observations one in each cluster (SAS,
1999). This method was also recommended by Cababasay (1991) due to its best overall
performance.
The PROC CLUSTER statement provided useful statistics (pseudo
F, pseudo t 2 and cubic clustering criteria) that can estimate the number of clusters that
will be formed out of a given data. They refer to the values that are measures of fit for
the analysis of each specified number of clusters (Callantes, 2003). These statistics
were given using the PSEUDO and CCC options, respectively, which are one of the
options used to control the printing of the cluster history (SAS, 1999).
With pseudo F statistic, Figure 3 reveals that the possible numbers
of clusters used to stratify the 69 accessions of Solanum spp. were 2 and 10. With the
pseudo t 2 statistic, however, Figure 4 shows that the accessions could be grouped in to
1 and 6 clusters. Figure 5, on the other hand, gave an estimate of 7 clusters using the
cubic clustering criterion. In this case, there was no consensus that could be found
among these criteria.
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