Add lagged versions of a longitudinal variable

Creates one or more lagged versions of a variable in long-format data. Lagged values are computed separately within each subject and added as new columns. The data must be sorted by id and visit time before calling this function.

Usage

add_lag_terms(df, var, lag = 1, fill = 0)

Arguments

df

A long-format data frame containing an `id` column identifying subjects.

var

Character string giving the name of the variable to lag.

lag

Integer vector specifying the lag(s) to create.

fill

Value used when a lagged observation is unavailable (e.g. at the first visit). Defaults to 0.

Value

The input data frame with additional columns named `(var)_lag_(lag)` containing the requested lagged values.

Examples

df <- data.frame(
  id = c(1, 1, 1, 2, 2, 2),
  visit_time = c(0, 1, 2, 0, 1, 2),
  A = c(0, 1, 1, 1, 0, 0)
)

# Add a 1-visit lag of A
add_lag_terms(df, "A")
#>   id visit_time A A_lag_1
#> 1  1          0 0       0
#> 2  1          1 1       0
#> 3  1          2 1       1
#> 4  2          0 1       0
#> 5  2          1 0       1
#> 6  2          2 0       0

# Add both 1- and 2-visit lags
add_lag_terms(df, "A", lag = c(1, 2))
#>   id visit_time A A_lag_1 A_lag_2
#> 1  1          0 0       0       0
#> 2  1          1 1       0       0
#> 3  1          2 1       1       0
#> 4  2          0 1       0       0
#> 5  2          1 0       1       0
#> 6  2          2 0       0       1