Critically ill patients are often given intravenous fluids in hospital, either in the form of balanced crystalloids or saline solutions. In a 2018 study published in The New England Journal of Medicine, researchers investigated which of these approaches resulted in better clinical outcomes. Read this excerpt from the abstract that accompanies this study and answer the following questions (Semmler et al. 2018).

Methods: In a pragmatic, cluster-randomized, multiple-crossover trial conducted in five intensive care units at an academic center, we assigned 15,802 adults to receive saline or balanced crystalloids. The primary outcome was a major adverse kidney event within 30 days — a composite of death from any cause, new renal-replacement therapy, or persistent renal dysfunction.

Results: Among the 7942 patients in the balanced-crystalloids group, 1139 (14.3%) had a major adverse kidney event, as compared with 1211 of 7860 patients (15.4%) in the saline group (P = 0.04).

a. Identify the treatment variable.

b. The response variable in this study is major adverse kidney event within 30 days. Was there a significant difference in occurrence of major adverse kidney events between the two groups? Explain. Assume a significance level of 0.05.

c. Based on this study, do you think one type of intravenous fluid may be preferable over the other? Explain.