%0 Journal Article
%T Competing risks models and time-dependent covariates
%A Adrian Barnett
%A Nick Graves
%J Critical Care
%D 2008
%I BioMed Central
%R 10.1186/cc6840
%X In the present issue of Critical Care Wolkewitz and colleagues use competing risks models to examine risk factors for nosocomial pneumonia and mortality in an intensive care unit [1]. Competing risks models offer significant advantages over standard survival analysis [2]. In a standard survival analysis there is one event (for example, death) and one time (for example, days until death). Often we have a set of covariates and want to know which are most predictive of the event. In competing risks models the number of events can be greater than one. In the study by Wolkewitz and colleagues there were three competing risks: nosocomial pneumonia, death and discharge. Covariates can depend on the competing risk. A good example from the study by Wolkewitz and colleagues is elective surgery before admission, which increased the risk of nosocomial pneumonia but decreased the risk of death and discharge.Competing risks models can incorporate time-dependent covariates using a Cox proportional hazards model. A time-dependent covariate is one that changes during the study period; for example, ventilation (yes/no). A time-independent covariate does not change; for example, sex. Time-dependent covariates can be richer than time-independent covariates because they offer the chance to examine the order of exposure and outcome [3]. The study by Wolkewitz and colleagues involved 10 binary time-dependent covariates, including nosocomial pneumonia.To use time-dependent covariates the data need to be arranged in a nonstandard format [2,4]. A new row needs to be added each time a covariate changes. As an example of a competing risks model, consider the subject presented in Table 1. When this subject entered the intensive care unit their start time was set to 0. After 2 days they contracted a nosocomial infection, and so their period with this pattern of covariates was censored. On day 3 the subject was ventilated, and so a new row is added to include this covariate pattern. After being v
%U http://ccforum.com/content/12/2/134