More correctly, for the same data, insertion sort may use fewer comparisons on average than does selection sort for the same data.
Insertion sort searches the sorted part of the array. It can, on average, stop after looking through half the elements, assuming a uniform distribution for data. Selection sort looks through the unsorted part of the array, which means it must search all the data. In the average case both sorts do O(n2) comparisons, but insertion sort should have a smaller constant than does selection sort.
This page last modified on 24 January 2006.