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TESI DOCTORAL - La Salle

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C.2. Estimation of the computationally optimal RHCA<br />

sets, our proposed method allows obtaining a pretty accurate estimation of the execution<br />

time of any serial RHCA variant which, at the same time, allows the user to make a reliable<br />

decision regarding the most computationally efficient consensus architecture whatever the<br />

diversity scenario and consensus function is employed.<br />

Secondly, figure C.12 presents the corresponding magnitudes in the case that the fully<br />

parallel implementations of RHCA are employed. In this situation, the estimation of the real<br />

execution time is not as accurate as in the serial case, although the running time deviation<br />

suffered when a suboptimal RHCA architecture is selected is, from a practical viewpoint,<br />

perfectly assumable — a fact that has already been reported in the previous data sets.<br />

C.2.7 MFeat data set<br />

In this section, the results of estimating the execution times of RHCA are compared to their<br />

real counterparts across four diversity scenarios in the context of the MFeat data collection.<br />

The cluster ensemble sizes corresponding to these diversity scenarios are l =6, 60, 114 and 168,<br />

respectively.<br />

Figure C.13 presents the estimated and real running times of multiple variants of the<br />

serial implementation of RHCA on this data set. Besides the notably high accuracy of<br />

the estimation, we would like to highlight that flat consensus turns out to be the most<br />

efficient consensus architecture in the four diversity scenarios for all but two of the consensus<br />

functions employed (MCLA and HGPA), a behaviour that has already been observed in<br />

other data collections with small cluster ensembles (e.g. the Iris data set).<br />

The results corresponding to the parallel implementation of RHCA are depicted in<br />

figure C.14. In this case, the use of the HGPA and MCLA consensus functions as clustering<br />

combiners also make the RHCA variants with s =2ands = 3 stages computationally<br />

optimal. However, for the remaining consensus functions, flat consensus mostly prevails as<br />

the most efficient consensus architecture in most diversity scenarios.<br />

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