Demand for intensive care unit (ICU) services is increasing [1], and at a rate that is higher than the average for all health care services [2]. Increase in treatment and monitoring technology, patients' expectations, and ageing population all contribute to this increased demand for intensive care services [1]. Indeed, intensive care is increasingly being provided to older and sicker patients, whom in the past were not treated in intensive care [3]. Intensive care services accounted for 10% of the US$2.1 trillion total health expenditures on health care in the United States in 2006 [4] and has been estimated to cost more than £700 million in the United Kingdom in 1999 [5]. The cost of intensive care services coupled with increasing demand provides the rationale for improved modelling of outcomes of critically ill patients.