Pre-calculated LCIs with uncertainties revisited

Abstract

Purpose

Heijungs et al. (Int J Life Cycle Assess 22(3):461, 2017) argued that (1) dependent sampling is required when analyzing uncertainties using Monte Carlo simulation in a comparative life cycle assessment (LCA) and that (2) using pre-calculated geometric standard deviations (GSDs) will lead to a “large overestimation” of the uncertainty of the results in a comparative LCA. While we agree with Heijungs and colleagues on the former from a theoretical standpoint, we find that the latter needs to be substantiated empirically. With this article, we tested the hypothesis that the use of pre-calculated life cycle inventories (LCIs) leads to an overestimation of uncertainty. In addition, we clarified a number of relevant issues and misconceptions in and around the independent sampling.

Methods

We compared the GSDs of LCIs for 100 randomly selected Ecoinvent processes that were derived (1) using pre-calculated GSD values (partially independent sampling PIS) and (2) without using pre-calculated values (fully dependent sampling FDS). We tested the null hypothesis, E(GSD ^FDS/GSD ^PIS) < 1, using a t test.

Results and discussion

The difference in the GSDs from fully dependent sampling and partially independent sampling (using pre-calculated LCIs) was statistically significant, though the average ratio GSD ^FDS/GSD ^PIS was very close to 1. Furthermore, the average ratio was larger than 1 with the confidence interval ranging from 1.0681 to 1.0696 at the 95% confidence level. Therefore, the null hypothesis was rejected.

Conclusions

According to our empirical analyses, the use of pre-calculated LCIs and associated GSDs leads to a slight underestimation rather than an overestimation—not to mention “large overestimation”—of uncertainties in the results. GSDs in the results derived from pre-calculated LCIs and associated GSDs are very close to those derived from fully dependent sampling, and therefore, in practice, pre-calculated LCIs can be used as a reasonable proxy in understanding the uncertainties and variabilities of both non-comparative and comparative LCAs. Given the high uncertainty in the uncertainty analysis itself, the minor difference between GSDs from pre-calculated values and fully dependent sampling should not prevent LCA practitioners from using pre-calculated LCIs and corresponding GSDs for uncertainty analysis when no better option is reasonably practicable.