3.2.1 QUALITATIVE AND QUANTITATIVE ASSESSMENT
The need to assess the safety or other performance of a Terminal Airspace design is one reason for establishing safety and performance criteria. Assessment is an ongoing process: qualitative assessment which begins at conceptualisation and continues through implementation also provides the foundation for quantitative assessment.
Two types of assessment have been distinguished: qualitative and quantitative assessment.
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Qualitative assessment is achieved by expert judgement being used to assess the design using ICAO standards, recommended practices and procedures as a benchmark. Qualitative assessment relies upon expert (air traffic control/operational) judgement and effectively forms the basis for the design concept (and the Critical Review of the Reference Scenario and the identification of Assumptions, Constraints and Enablers). Qualitative Assessment is an on-going process: as well as providing the basis for the design concept, this expert judgement is also used to qualitatively assess all phases of the design methodology, and it is integral to quantitative assessment and to safety measurement – even when the emphasis appears to be on measurement against an absolute threshold. That qualitative assessment forms the backbone of the various validation methodologies will become evident in Part D, and it is used in implementation planning (Part E).

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In contrast, Quantitative assessment is concerned with ‘quantified’ results produced in the form of numerical data. e.g. capacity increased by 20%.

It is perhaps because quantitative assessment appears to provide ‘tangible’ values that these results are perceived as being preferable to those of a qualitative nature. But this perception inaccurate – for at least two reasons:
[i]  Qualitative assessment made by expert ATC judgement is the primarily way in which ICAO SARPs and procedures are safe-guarded during the design process; and
[ii]  if total reliance is placed upon quantitative results without qualitatively assessing what they mean (i.e. using expert judgement to interpret the results), the value of the quantitative assessment is likely to be less.
[iii]  Quantitative assessments are inadequate in effectively depicting and quantifying the complex and highly variable nature of airspace and air traffic operations. This is because quantitative safety assessment models tend to simplify many operational elements in order to be manageable. This results in limiting the number of elements to those having the greatest potential for effect – and this can return incorrect results. For this reason, quantitative assessment needs to be balanced by qualitative assessment i.e. operational judgment and experience for the complex interactions, conditions, dependencies and mitigations for which quantitative assessment cannot provide a meaningful measure.
What will become evident in the next section is that both qualitative and quantitative assessment are essential to the process of safety evaluation.
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3.2.2 EVALUATING SAFETY
ICAO Annex 11 and PANS-ATM includes requirements for a Safety Assessment to be undertaken when making certain modifications to the Air Traffic Management System. Significantly, ICAO has detailed those instances in which a Safety Assessment is required and an excerpt from the relevant ICAO material has been included in Part A of this document at Chapter 2. Because airspace designers must ensure and demonstrate that an airspace design is safe2 (i.e. provide evidence of safety through a safety assessment process), this section provides a broad overview of how safety can be evaluated.
　
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