The Collaborative International Dictionary of English v.0.48:

Effectiveness \Ef*fect"ive*ness\, n.
   The quality of being effective.
   [1913 Webster]

WordNet (r) 3.0 (2006):

effectiveness
    n 1: power to be effective; the quality of being able to bring
         about an effect [syn: effectiveness, effectivity,
         effectualness, effectuality] [ant: ineffectiveness,
         ineffectuality, ineffectualness]
    2: capacity to produce strong physiological or chemical effects;
       "the toxin's potency"; "the strength of the drinks" [syn:
       potency, effectiveness, strength]

Moby Thesaurus II by Grady Ward, 1.0:

139 Moby Thesaurus words for "effectiveness":
   acme, amperage, applicability, armipotence, authority,
   authorization, availability, be-all and end-all, beef, bite,
   bitingness, black power, blue ribbon, brute force, capability,
   championship, charge, charisma, clout, cogence, cogency, command,
   compulsion, control, cuttingness, dint, directorship, dominion,
   drive, duress, effect, effectuality, efficacy, efficiency, energy,
   first place, first prize, flower power, force, force majeure,
   forcefulness, full blast, full force, functionality, guts,
   headship, hegemony, height, helpfulness, highest, imperium,
   impressiveness, incisiveness, influence, jurisdiction, kingship,
   leadership, lordship, main force, main strength, mana, management,
   mastership, mastery, maximum, might, might and main, mightiness,
   mordancy, most, moxie, muscle power, ne plus ultra, nervosity,
   nervousness, new high, operability, palms, paramountcy,
   performance, pizzazz, poignancy, poop, potence, potency,
   potentiality, power, power pack, power structure, power struggle,
   powerfulness, practicability, practical utility, practicality,
   prepotency, presidency, primacy, productiveness, productivity,
   profitability, puissance, pull, punch, push, record, rule, say,
   serviceability, sinew, sinewiness, sovereignty, steam, strength,
   strong arm, strong language, superiority, superpower, supremacy,
   sway, top spot, trenchancy, usability, use, usefulness, utility,
   utilizability, validity, vehemence, verve, vigor, vigorousness,
   vim, virility, virtue, virulence, vitality, wattage, weight,
   zenith