Idiom (compared/comparison)

[sathyanarayanar]

I have a doubt regarding the idiom "compared/comparison" , which is given in the Manhattan SC book.

It says :

WRONG : When compared to horses , zebras are vicious.

But is this wrong because of the word "When" in the beginning of the sentence ?
Is it right to say " Compared to horses, zebras are vicious".
If it is wrong because of the word "when" , can I generalize this to all GMAT questions containing "when compared to " ?

[donnabrenton]

When compared with horses , zebras are vicious.

[sathyanarayanar]

But it says in the book that GMAT ignores the traditional difference between "compared to " and "compared with " and that it considers both these expressions to be the same ?

[tim]

yeah, apparently "compared to" and "compared with" can both be okay, but you don't want to use a "when" with either one..