Authors have seen EIA effectiveness to refer to the quality of EIA reports and procedural follow up, likewise the benefit of EIA in development planning as measured either quantitatively or qualitatively.
For authors such as Sadler, major aspects of effectiveness are the quality of EIA reports, effects on decision making through the management of impacts, and post auditing.
I have research work published here on Research gate on EIA effectiveness in Nigeria's study area. In that research, I assessed EIA's efficacy, and what constitutes EIA effectiveness, how it can at least be measured. Kindly see the document attached o my Reserachgate profile. Check the link below
Article The Effectiveness and Relevance of Environmental Impact Asse...
There are different ways for measuring effectiveness. See e.g.:
Retief, F.; Fischer, T. B.; Alberts, R.; Roos, C. & Cilliers, D. 2020. An administrative justice perspective on improving EIA effectiveness, Impact Assessment and Project Appraisal, 38(2): 151-155
Arts, J.; Runhaar, H.; Fischer, T. B.; Jha-Thakur, U.; van Laerhoven, F.; Driessen, P. & Onyango, V. 2012. The Effectiveness of EIA as an Instrument for Environmental Governance – A Comparison of the Nether-lands and the UK, Journal of Environmental Assessment Policy and Management, 14(4): 1250025-1-40.
and on SEA:
Fischer, T. B. & Retief, F. 2021. Does Strategic Environmental Assessment Lead to More Environmentally Sustainable Decisions? – Reflections on Its Substantive Effectiveness, in: Fischer, T. B. & González, A. (eds.). Handbook on Strategic Environmental Assessment, Edward Elgar (chapter 8).
at times effectiveness is expressed through other terms, e.g. performance:
Fischer, T. B. 2002. SEA performance criteria – the same requirements for every assessment? Journal of Environmental Assessment Policy and Management, 4(1): 83-99.