The effectiveness of EA is defined as the degree to which EA has been successful in meeting its objectives and purpose. The ability of EA in meeting its objectives and purpose depends on several interlinked factors. In the current thesis some of the factors, such as the quality of EA documentation and process are addressed. The quality evaluation of 50 Environmental Impact Statements (EIS) demonstrated a generally satisfactory level of quality, but it varied between different EIS and topic areas.

Most gaps were identified in the project description, mitigation measures and consideration of alternatives. The experiments with individual and group assessments demonstrated a large individual variability of results and herewith the highly subjective nature of the assessment. The group evaluations resulted in several topic areas of EIS receiving lower grades than the initial individual assessments. It was speculated that a group of reviewers produced a more diverse competence and thus scrutinized the EIS more severely than an individual assessor who might not have been competent in all areas of environmental effects. Screening of projects and plans has an important role in putting the precautionary principle into action.

However, the research results demonstrated that, for example, screening of likely adverse effects on Natura 2000 sites was poorly conducted and justified. Only every sixth screening decision addressed such effects. If addressed, then only the distance of the proposed development in relation to the Natura 2000 site was considered, ignoring the type and significance of effects.

The study of the EA process showed that participants and stakeholders had different expectations regarding the management and outcome of the process. It is proposed that the objectives and the outcome of the EA need to be agreed at the onset of the process, and communicated effectively throughout the EIA process. Based on the literature review and the current research, five interlinked and interdependent components (drivers) characteristic of an effective EA could be derived:

  1. clearly defined objectives and purpose of EA,
  2. effective participatory and discursive process throughout the EA process,
  3. high quality documentation,
  4. positive effect of EA on the design of the project or plan (i.e. integration of EA and planning process) and on decision making, and verified by follow-up, and
  5. highly time and cost effective. The level of application of these components, i.e. the effectiveness of EA depends on the legal regulation and guidance provided for best practice, but also on sufficient administrative capacities and expertise to produce environmental benefit.

The applicability of the theoretical framework of other drivers of EA effectiveness, in addition to the quality of EA documentation and EA process studied in this thesis, however, needs to be further elaborated and tested in practice.