Define a survival process

Records one Weibull hazard segment for one event time. The simstudy parameterization is used throughout: an event time drawn from this segment has survival function S(t) = exp(-exp(formula) * t^(1 / shape) / scale), so shape is the reciprocal of the textbook Weibull shape, and exp(formula) multiplies the whole cumulative hazard. A coefficient inside formula is therefore a log hazard ratio, while formula itself equals the log hazard only when shape and scale are both 1.

Usage

define_survival(event, formula = 0, scale = 1, shape = 1, transition = 0)

Arguments

event

Name of the event-time variable. A single non-empty string.

formula

Linear predictor on the log scale, given as a number, a string, or an unquoted expression over the covariate columns (for example "-8 + 0.5 * treatment"). Defaults to 0. It is stored as text and evaluated row by row against the data.

scale

Positive Weibull scale, as a number, a string, or an expression over the covariates. Defaults to 1. It divides the cumulative hazard.

shape

Positive Weibull shape in the simstudy parameterization (the exponent applied to the transformed time, so the hazard is constant when shape is 1 and decreasing when shape is above 1). Defaults to 1.

transition

Time at which this hazard segment begins. A single finite non-negative number, defaulting to 0. The first segment of an event must start at 0.

Value

A one-row simulab_survival_spec base data.frame with columns event, formula, scale and shape (all character, the stored definition text) and the numeric column transition.

Examples

define_survival("time", formula = -8, shape = 0.3)
#>   event formula scale shape transition
#> 1  time      -8     1   0.3          0