Table 1. ICU Capacity Strain Variables and Definitions.

Variable*	Operational Definition	Method of Calculation
Census†	Standardized ICU census	The difference between the number of patients spending at least 2 h in the ICU that day (12:00 a.m.–11:59 p.m.) and the mean census for that year, divided by the yearly SD of that ICU's census.
Admissions	Proportion of new admissions	The number of new admissions on that day divided by the raw (nonstandardized) ICU census that day.
Acuity‡	Average predicted probability of death of the other patients in the ICU	The sum of the probability of death of other ICU patients divided by the number of other patients in the ICU on that day, excluding the index patient.

ICU = intensive care unit.

^*All candidate capacity strain variables are measured on the index patient's day of ICU discharge. Calculations included all patients (including those who died in the ICU) who were present on the day of an index patient's discharge.

^†A 2-h cutoff was chosen to account for possible coding errors in bed assignment that were quickly corrected.

^‡Probabilities of death derived from Mortality Probability Model III scores.