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I have unlabeled dataset and I'm using the hierarchical clustering to generate a groups from this data. I had a look to the lit and I found that there are two approaches to evaluate the clustering results. (1). Internal Validation, (2) External Validation.
How can I verify that my data clustered well? Is using one of the Internal Validation such as "Sum of Squared Error" enough to say my clustering system perform well? or do I need to apply different criterias?