Much of works in this field has been conducted in the first major international project on the effectiveness of international environmental regimes, led by Ed Miles and Arild Underdal.
Here is the link for book:
Chapter The Effectiveness of Environmental Policies
The effectiveness of environmental measures/policy in the field of human health impact is measured by the WHO method: lost years of life due to illness, disability or death.
The best way to measure effectiveness is to measure whether and to what extent the effect was achieved. See? for example, OECD principles on water governance http://www.oecd.org/governance/oecd-principles-on-water-governance.htm
Its depend on execution of environment policies to justify the role. EIA is a tool to asses the maximum extent impact of a project on environment but corrective and suggestive measures are need to be respected as per policy document by the state or organisation.
As my opinion we should first development the indicator of the policy achievement. Effectiveness can define how good is the policy planned and how far it's achive the target. You can use mathematical formula.