Page 46 - Materials Chemistry, Second Edition

AN OVERVIEW OF THE LIFE CYCLE ASSESSMENT METHOD 27

depletion. To put these results in perspective, the normalization expresses
them as a share of the total impact size in the region. Arbitrary differences
due to a choice of units disappear, and it becomes clear to which impact
category a product contributes relatively much. The units of the normalize
indicator results are equal; nevertheless, such numbers cannot meaning-
fully be added because the severity of the different impact categories has not
yet been accounted for. This can be done in the weighting step; see below.
Normalization fulfills several functions: it provides insight into the meaning
of the impact indicator results, it helps to check for errors, and it prepares for
a possible weighting step.
Grouping is another optional step, although it is seldom seen in LCA studies.
ISO defines it as "the assignment of impact categories into one or more sets."
ISO mentions two ways:

• Sorting (on a nominal basis, like global/regional/local);
• Ranking (on an ordinal basis, like high/medium/low priority).

Weighting, like characterization, converts and aggregates, but while charac-
terization does so for the LCI results, weighting starts with the characteriza-
tion (or normalization) results. Typically, weighting factors are applied, either
to the characterization indicator results, or to their normalized version. The
weighting factors themselves are supposed to reflect value judgements, such
as social and political priorities. Weighting typically produces one final num-
ber, by means of:

W = £WF c x7 c
c

where I c again symbolizes the impact score (or normalized impact score)
for impact category c, WF c the weighting factor for this impact category, and
W the weighted result. Well-known examples of such weighted results are the
eco-indicator and the ELU (environmental load unit).
The data quality analysis, finally, relates to an analysis of completeness,
uncertainties, etc. We treat this more systematically in the section on interpre-
tation below.

2.1.4 Interpretation

ISO defines interpretation as the "phase of life cycle assessment in which the
findings of either the inventory analysis or the impact assessment, or both, are
evaluated in relation to the defined goal and scope in order to reach conclu-
sions and recommendations." Several elements are mentioned by ISO:

• Identification of significant issues;
• An evaluation that considers completeness, sensitivity and
consistency checks;