There's a table that records the most skipped objects already, and these are likely to be the most challenging objects. Current figures range from 0 to 110 - some of this will relate to how long the images have been in the database, but it might also provide useful divisions - after the second round, you get a few slightly difficult objects; after five you get more difficult objects, after ten you get the most difficult.
ie you should get more challenging objects.
There's a table that records the most skipped objects already, and these are likely to be the most challenging objects. Current figures range from 0 to 110 - some of this will relate to how long the images have been in the database, but it might also provide useful divisions - after the second round, you get a few slightly difficult objects; after five you get more difficult objects, after ten you get the most difficult.