EIA is generally conduced in tiers. In most countries, EIA processes fall into four tiers.
Screening: decides whether the EIA process is applicable at all to the development project. Screening should basically “clear” all those projects where there are no major negative impact issues.
Scoping: helps in understanding the extent of environmental impacts and identifies significant environmental issues for further study. In the activity-component framework, this would mean that scoping should assist in the identification of impacting activities and impacted environmental components for all major negative impact issues.
Initial environmental examination: assesses the severity of the significant issues and finds ways to mitigate or enhance environmental impacts by considering the available information from past experience or the standard operating practices (SOPs; standard mitigation measures).
Detailed or comprehensive EIA: carries out a detailed examination of impacts by conducting relevant surveys. Studies are monitored by applying more rigorous impact evaluation/prediction tools where necessary and ensuring the effectiveness of the mitigation and enhancement measures. Detailed EIA thus focuses on impacts which are critical but not fully understood.
In a tiered EIA process, such as that above, the relevant regulatory agency decides whether there is nothing to be concerned about, or that the evaluation should proceed to the next tier.
Conducting EIA in tiers helps optimize the resources as well as to increase the effectiveness of the exercise by maintaining a better focus. Another advantage of a tiered approach is that the extent of the inquiry or examination expands with the advancing development of the project plans. Such a tiered process is most suited for developing countries which are in the rapid phase of development.