Statistical Assumptions: (http://en.wikipedia.org/wiki/Statistical_assumption)

1. In practice more than 31 items are necessary before any statistic can be calculated.
2. Random variables are IID - Independent, Identically distributed.
3. Central Limit Theorem - in larger sample populations tend to be a Normal distribution.
4. ... see Wikipedia and it's associated references

This means that the pictures of the items on the previous page are "undefined".

The dilemma is that humans can easily recognize these objects.