# Create a bootstrap sample of persons (as identified by `lex.id`) from a Lexis object `lex.id` is the person identifier in a `Lexis` object. This is used to sample persons from a Lexis object. If a person is sampled, all records from this persons is transported to the bootstrap sample. ```r nid( Lx, ... ) ## S3 method for class 'Lexis' nid( Lx, by=NULL, ... ) bootLexis( Lx, size = NULL, by = NULL, replace=TRUE ) ``` ## Arguments - `Lx`: A `Lexis` object. - ``...``: Parameters passed on to other methods. - `size`: Numeric. How many persons should be sampled from the `Lexis` object. Defaults to the number of persons in the `Lx`, or, if `by` is given, to the number of persons in each level of `by`. If `by` is given, `size` can have length `length(unique(by))`, to indicate how many are sampled from each level of `by`. - `by`: Character. Name of a variable (converted to factor) in the `Lexis` object. Bootstrap sampling is done within each level of by. Calculation of the number of persons (`lex.id`) is done within each level of `by`, and a vector returned. - `replace`: Should persons be sampled by replacement? Default is `TRUE`. Setting `replace` to `FALSE` enables selecting a random subset of persons from the Lexis object. ## Returns `bootLexis` returns a Lexis object of the same structure as the input, with **persons** bootstrapped. The variable `lex.id` in the resulting `Lexis` object has values 1,2,... The original values of `lex.id` from `Lx` are stored in the variable `old.id`. `nid` counts the number of persons in a Lexis object, possibly by `by`. If `by` is given, a named vector is returned. ## Author(s) Bendix Carstensen, [http://bendixcarstensen.com](http://bendixcarstensen.com). ## See Also `Relevel.Lexis`,`subset.Lexis` ## Examples ```r # A small bogus cohort xcoh <- data.frame( id = c("A", "B", "C"), birth = c("1952-07-14", "1954-04-01", "1987-06-10"), entry = c("1965-08-04", "1972-09-08", "1991-12-23"), exit = c("1997-06-27", "1995-05-23", "1998-07-24"), fail = c(1, 0, 1), sex = c("M","F","M") ) # Convert to calendar years for( i in 2:4 ) xcoh[,i] <- cal.yr(xcoh[,i]) xcoh <- xcoh[sample(1:3, 10, replace = TRUE),] xcoh$entry <- xcoh$entry + runif(10, 0, 10) xcoh$exit <- xcoh$entry + runif(10, 0, 10) Lcoh <- Lexis(entry = list(per = entry), exit = list(per = exit, age = exit - birth), exit.status = fail, data = xcoh) Lcoh Lx <- splitLexis(Lcoh, breaks = 0:10 * 10, "age") Lx nid(Lx) nid(Lx, by="sex") Lb <- bootLexis(Lx) head(Lb) nid(bootLexis(Lx, size = 7)) Li <- bootLexis(Lx, by = "id") # superfluous summary(Lx) summary(Li) L2 <- bootLexis(Lx, by = "sex", size = c(2, 5)) nid(L2, by = "sex") summary(L2, by = "sex") ```

bootLexis function