Sadler (1994) commented that in order to be an effective instrument for sustainable development EIA should be tiered in the following way.
1. Screening economic and development policies for their conformity with sustainability goals and principles.
2. Preliminary assessment of environmental costs of development programmes to identify low-impact, resource efficient energy, transportation and other sector strategies.
3. Area wise assessment to establish resource values and land-use capabilities for regional development.
4. Project EIA to identify in-kind compensation for natural capital losses (e.g. Affecting residual fish and wildlife losses by ex-situ habitat rehabilitation and enhancement).

While admitting the logical strength of the above framework it must be noted that the conditions for implementing a tiered approach either may not be in place or only partially developed. Moreover at the project level the greatest qualitative enhancement that can be achieved through the tiered approach is by establishing initial benchmark against which reviews can be conducted. However, considering the prevailing constraints of information, skills and resources in developing countries this study considers strategic EIA a distant goal and restricts itself to betterment of project EIAs in order to achieve the goals of environmental sustainability. In addition, the author wishes to point out that the notion of safe minimum standard to maintain the assimilative and regenerative capacity of the environment is well-established (e.g. ambient air and water quality indices). For the time being such standards may substitute the environmental bench marking job. In order to achieve environmental sustainability the focus shift required in EIA is from the currently practised 'impact minimisation' to 'impact compensation'. While impact minimisation approach essentially attempts to reduce the adverse effects of major human activities through mitigation and management, the impact compensation refers to in-kind or comparable offsetting for estimated resource losses or environmental damages (Sadler 1996). Only this shift in the principal focus will convert EIA to an appropriate instrument for planning 'environmental sustainability. Impact compensation focus can be attained in EIA through incorporation of necessary shift in the key steps (e.g. screening, scoping, impact analysis, mitigation and monitoring) towards more positive examination of alternatives and impact compensation possibilities to ensure maintenance of natural capital. It will not, therefore, involve any significant structural change to EIA process and procedure.

3.2 Other Related Definitions
3.2.1 Environmental Sustainability and Sustainable Development
The previous chapter outlined the concept of environmental sustainability. If the WCED (1987) definition of sustainable development is accepted, it may be argued that some of the assumptions and preconditions do not allow sustainable development to be operationalised at the present state of development. Therivel et al. (1992) and Goodland (1994) suggest that environmental management should aim to achieve environmental sustainability rather than sustainable development. It is beyond the scope of this study to make finer distinction between the two. While, in general, environmental sustainability is considered the principal goal to be achieved by EIA, at times the terms 'environmental sustainability' and 'sustainable development' have been used interchangeably in this study.

3.2.2 Strategic Environmental Assessment
Lee and Walsh (1992) defined strategic environmental assessment (SEA) as the environmental impact assessment process for policies, plans and programs that are approved earlier than the authorisation of individual projects. This definition has been retained by Canter (1996) and Gilpin (1994). Therivel et al. (1992) defined SEA as 'the formalised, systematic and comprehensive process of evaluating the environmental impacts of a policy plan or programme and its alternatives, including the preparation of a written report on the findings of that evaluation, and using the findings in public accountable decision making'. This study supports the definition by Therivel et al. (1992).

Over the last few years SEA has emerged more as a concept than as a tool. SEA seeks to provide a framework within which individual EIAs can take place. The framework takes into account, inter-alia cumulative effects, greenhouse policies, resource conservation and environmental sustainability (Gilpin 1994).

3.2.3 Risk Assessment
Risk assessment involves quantification of probability and uncertainty. According to Covello and Mumpower (1985) risk assessment is a study that provide quantitative measures of risk levels, where risk refers to the possibility of uncertain adverse consequences. Risk assessment attempts to estimate the probability of occurrence of hazardous events as consequence of an activity. It also provides an assessment of the population at risk of exposure and the possible adverse consequences (Andrews 1988).

3.2.4 Cumulative Effects Assessment
Growing attention is being paid to the developments in the theory and practice of cumulative effects assessment (CEA) principally because of an increasing realisation that perhaps 'the most ecologically devastating environmental effects may not result from the direct effects of a particular proposal, but from the combination of existing stresses and the individually minor effects of multiple actions over time' (Clark 1994). The increased attention is also due to current policy and regulatory requirements in countries such as Canada and the United States (Smit and Spalling 1995). In the United States the terms effects and impacts are considered synonymous (Canter 1996) and the term cumulative impact is defined as 'the impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions regardless of what agency (federal or non federal) or person undertakes such other actions. Cumulative impacts can result from individually minor but collectively significant actions taking place over a period of time' (USCEQ 1987). The four basic pathways that contribute to cumulative effects are generally recognised as additive, interactive, compounding or synergistic (Cada and Hunsaker 1990, Clark 1994). Cumulative effects assessment (CEA) may be defined as the process of systematically analysing and evaluating environmental change (Spalling 1994, Smit and Spalling 1995). The concept of CEA is built upon conventional EIA. Only the focus is on the combined impacts of multiple activities, rather than the effects of specific developmental activities. CEA provides a more comprehensive and holistic evaluation framework than conventional EIA (Roe et al. 1995).