In microarray data analysis, visualizations based on agglomerative clustering results are widely applied to help biomedical researchers in generating a mental model of their data. In order to support a selection of the to-be-applied algorithm and parameterizations, we propose a novel cluster index, the
tree index (TI)
, to evaluate hierarchical cluster results regarding their visual appearance and their accordance to available background information. Visually appealing cluster trees are characterized by splits that separate those homogeneous clusters from the rest of the data, which have low inner cluster variance and share a medical class label. To evaluate clustering trees regarding this property, the TI computes the likeliness of every single split in the cluster tree. Computing TIs for different algorithms and parameterizations allows to identify the most appealing cluster tree among many possible tree visualizations obtained. Application is shown on simulated data as well as on two public available cancer data sets.