When you construct a comparison, you must make the twoitems being compared parallel in structure.
Ecological modeling [nounphrase] is even more difficult than to construct a hydrologicalmodel[infinitive phrase].
Ecological modeling [noun phrase] is even more difficultthan hydrological modeling [noun phrase].
--Norman Boucher, "Back to the Everglades," TechnologyReview (modified)
The McGraw-Hill Companies, Inc..