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Person Years

Usage

pyears(formula, data, weights, subset, na.action, ratetable=survexp.us,
scale=365.25, model=F, x=F, y=F)

Arguments

formula a formula object. The response variable will be a vector of follow-up times for each subject, or a Surv object containing the survival time and an event indicator. The predictors consist of optional grouping variables separated by + operators (exactly as in survfit), time-dependent grouping variables such as age (specified with tcut), and optionally a ratetable() term. This latter matches each subject to his/her expected cohort.
data, as in other modeling routines. Weights are case weights.
ratetable a table of event rates, such as survexp.uswhite.
scale a scaling for the results. As most rate tables are in units/day, the default value of 365.25 causes the output to be reported in years.
model, flags to control what is returned. If any of these is true, then the model frame, the model matrix, and/or the vector of response times will be returned as components of the final result, with the same names as the flag arguments.

Description

Because pyears may have several time variables, it is necessary that all of them be in the same units. For instance in the call py <- pyears(futime ~ rx + ratetable(age=age, sex=sex, year=entry.dt)) with a ratetable whose natural unit is days, it is important that futime, age and entry.dt all be in days. Given the wide range of possible inputs, it is difficult for the routine to do sanity checks of this aspect. A special function tcut is needed to specify time-dependent cutpoints. For instance, assume that age is in years, and that the desired final arrays have as one of their margins the age groups 0-2, 2-10, 10-25, and 25+. A subject who enters the study at age 4 and remains under observation for 10 years will contribute follow-up time to both the 2-10 and 10-25 subsets. If cut(age, c(0,2,10,25,100)) were used in the formula, the subject would be classifed according to his starting age only. The tcut function has the same arguments as cut, but produces a different output object which allows the pyears function to correctly track the subject. The results of pyears() are normally used as input to further calculations. The print routine, therefore, is designed to give only a summary of the table.

Value

a list with components

pyears an array containing the person-years of exposure. (Or other units, depending on the rate table and the scale).
n an array containing the number of subjects who contribute time to each cell of the pyears array.
event an array containing the observed number of events. This will be present only if the resonse variable is a Surv object.
expeced an array containing the expected number of events. This will be present only if there was a ratetable term.
offtable the number of person-years of exposure in the cohort that was not part of any cell in the pyears array. This is often useful as an error check; if there is a mismatch of units between two variables, nearly all the person years may be off table.
call an image of the call to the function.
na.action the na.action attribute contributed by an na.action routine, if any.

See Also

ratetable, survexp, Surv