I read this comment from another post - and thought to open a separate question:  

Bucket sort is more efficient for 'Dense' arrays, while Radix Sort can handle sparse (well, not exactly sparse, but spaced-out) arrays well.  

Please help understand how this is so?  

As far as I understand ,the 'denseness' of the population will impact the number of buckets equally for both the algorithms.  

Also,insertion sort (at each bucket) does not get much impacted by the density - or does it?