Create a Survival Object

DESCRIPTION:

Create a survival object, usually used as a response variable in a model formula.

USAGE:

Surv(time, event) 
Surv(time, time2, event, type=<<see below>>, origin=0) 
is.Surv(x) 

REQUIRED ARGUMENTS:

time
for right censored data, this is the follow up time. For interval data, the first argument is the starting time for the interval.
x
any S-PLUS object.

OPTIONAL ARGUMENTS:

time2
ending time of the interval for interval censored or counting process data only. Intervals are assumed to be open on the left and closed on the right, (start, end]. For counting process data, event indicates whether an event occurred at the end of the interval.
event
status indicator, normally 0 = alive and 1 = dead. Other choices are T/F (T = death) or 1/2 (2 = death). For interval censored data, the status indicator is 0 = right censored, 1 = event at time, 2 = left censored, and 3 = interval censored. For left-censored data, the status indicator is T=uncensored, F=censored. Although unusual, the event indicator can be omitted, in which case all subjects are assumed to have an event.
type
character string specifying the type of censoring. Possible values are "right", "left", "counting", "interval", or "interval2". The default is "right" or "counting" depending on whether the time2 argument is absent or present, respectively.
origin
vector or value to be subtracted from the time values. This option was intended for resetting the time origin in the case of time-dependent strata, but has rarely proven to be useful.

VALUE:

in the case of Surv, a matrix of 2 or 3 columns of class "Surv" containing time, time2 (if provided), and status.

In the case of is.Surv, a logical value T if x inherits from class "Surv" , otherwise an F.

SUBSCRIPTING:

Surv objects can be subscripted either as an object, e.g. x[1:3] using a single subscript; in which case the drop argument is ignored; or as a matrix, using two arguments. If the second subscript is missing and drop=F, the result of the subscripting will be a Surv object, e.g., x[1:3,,drop=F] , otherwise the result will be a matrix (or vector), in accordance with the default behavior for subscripting matrices.

DETAILS:

In theory it is possible to represent interval censored data without a third column containing the explicit status. Exact, right censored, left censored and interval censored observation would be represented as intervals of [a,a], [a, infinity), (-infinity,b], and [a,b) respectively; each interval is a pair of time points within which the event is known to have occurred.

If type="interval2", the representation given above is assumed, with NA taking the place of infinity. If type="interval", event must be given. If event is 0, 1 , or 2, the relevant information is assumed to be contained in time, the value in time2 is ignored, and the second column of the result contains a placeholder.

Presently, the only methods allowing interval censored data are the parametric models computed by survReg, so the distinction between open and closed intervals is unimportant. The distinction is important for counting process data and the Cox model.

The function tries to distinguish between the use of 0/1 and 1/2 coding for left and right censored data using if(max(status)==2) . If 1/2 coding is used and all the subjects are censored, it will guess wrong. Use 0/1 coding in this case.

SEE ALSO:

, , .

EXAMPLES:

Surv(leukemia$time, leukemia$status) 
Surv(heart$start, heart$stop, heart$event)