The change of rank relative to the previous time interval.

For example, [on the week of 2010 Aug 23rd](http://stackexchange.com/leagues/1/week/stackoverflow/2010-08-23), Pekka's rank was #7, and on the [previous week](http://stackexchange.com/leagues/1/week/stackoverflow/2010-08-16) his rank was #23, so the change is 23 − 7 = +16.