For example, in comparing capacitive deionization and reverse osmosis (RO) for brackish water desalination, pretreatment and brine disposal were excluded from one study because of their high sensitivities to variables (feedwater composition and geographical location, respectively) that were not relevant to the objectives of the study. Excluding these unit operations allowed the researchers to focus on core comparisons between capacitive deionization and RO without introducing additional unknowns that were expected to be independent of desalination technology choice (e.g., brine disposal method). Therefore, the definition of the conceptual system can be an iterative process to maintain focus on unit processes that are relevant to the sustainability indicators of interest or the targeted insight.