see Hall and Skinner in the appendix of your gifted child... as someone noted... it isn't the quality but the details. to steal from the book:

draws person with 2 parts - average is 48m (33.6m is 30% more advanced)

draws person with neck, hands, clothes - average is 72m (50.4 - considered 30% more advanced).

.. see book for more...

Also see Gifted Children by Ellen Winner - more on Art.

I don't know if any of this is valid but it is published.

C.