Sometimes it's necessary to sort sets of data based on metrics that are not recognized by a computer such as artistic quality of a picture. For example you can sort a set of numbers in increasing or decreasing order easily because each element in the set is comparable with another in a manner the computer can understand. For example

Given Set
5 6 4 7 3 8 2 9 1

Sort by Increasing Order

1 2 3 4 5 6 7 8 9
This is possible for a computer to do automatically because it can compare two elements and say which one is greater (4 is greater than 2).

But how can you let the computer sort pictures or audio clips based on their aesthetic quality?

Some websites and applications accomplish this by letting humans rate a picture in a scale of 1 to 10. This works to some extent, but when you sort it according to rating, one could easily point out two items out of order because different people's 1 to 10 scale is different. A alternative to this is applying comparison sorting techniques for pictures or audio clips. For example