The norm for LCA, ISO 14040 & 14044 describes the calculation sequence in 4 steps, see Fig. 5.2b.
At the previous webpage, LCA the basics, the first two steps have already been described, in general terms of the tasks for the LCA practitioner. In the 3rd step of “Fast Track” LCA, the practitioner can directly apply excel tables, since the indicator has already been chosen in the 1rst step in line with the goal of the study (as is required by the ISO14044). In the classical, rigorous approach, however, the choice of the indicator is postponed. It requires computer software add-up all the LCIs from the sub-systems first, and then combine all the substances from the total LCI in midpoints and endpoints. This process is depicted in Fig. 5.2c. and has the advantage that the practitioner can switch from indicator at this late stage of the procedure. It has the disadvantage, however, that the practitioner can easily change the indicator when he or she thinks that the conclusion is not convenient.