From Wiktionary, the free dictionary

From effective +‎ -ness.

• (weak vowel distinction) IPA^(key): /ɪˈfɛk.tɪv.nɪs/
• (weak vowel merger) IPA^(key): /əˈfɛk.tɪv.nəs/

effectiveness (countable and uncountable, plural effectivenesses)

1. The property of being effective, of achieving results.

   The effectiveness of the drug was well established.

2. The capacity or potential for achieving results.

   • 2013 August 31, Jack L. Goldsmith, “What Happened to the Rule of Law?”, in The New York Times:

     The problem with these precedents is that the Security Council did not authorize intervention in Syria, and an intervention there will thus harm, not help, the Council’s credibility and effectiveness.

3. The degree to which something achieves results.

   He questioned the effectiveness of the treatment.

   • 2013 September 1, Phil McNulty, “Liverpool 1-0 Man Utd”, in BBC Sport‎^[1], archived from the original on 16 July 2018:

     United were having more possession but a sign of the effectiveness of Liverpool's defence was that it took the visitors 76 minutes to force Mignolet into serious action, when he dived to punch away a shot from substitute Nani.

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