I am surprised about this hierarchical cluster! For me, it looks somehow abnormal. Or, maybe normal but I am not able to identify why it looks like this. Any idea why data would be clustered in such a structure?
This happens if:
- There is no clear way to separate the clusters, i.e. there is only one large group of instances and the instances which are far from this group are too far from each other to form their own cluster.
- The linkage criterion plays a big role: this is more likely to happen with single linkage clustering , because as the main cluster grows it becomes more likely to be the closest point to any other instance, therefore all the instances are "attached" to this cluster one by one. This is less likely to happen with complete-linkage clustering, but this option has other biases.