"BMI for dummies"
...and yet, it still doesn't cater for Dave.
I'm trying to think of the right word or phrase...
Inept, challenged, special, limited, thick, dim, dense, dull, slow, vacant, obtuse, witless, brainless, mindless, numbskull, blockhead, dunderhead, boneheaded, half-witted, empty-headed, clueless, hopeless, ineffectual, incompetent, useless, deficient, feeble-minded, subpar, sub-intellectual, non-functioning, dimwitted, simple, unique, gifted, exceptional, lights-on-nobody-home, sandwich-short-of-a-picnic, missing-a-few-marbles, thick-as-two-short-planks
...but I just can't seem to land on one that would actually fit.