Identify which units were trimmed

is_unit_trimmed() returns a logical vector indicating which observations were removed by trimming. This is a per-unit query, as opposed to is_ps_trimmed(), which tests whether the object has been trimmed at all.

The answer comes from the trimming record, which is written for a fixed set of observations and can both be lost and outlive them. On a ps_trim, vctrs::vec_slice() and c() drop it, and subassignment that grows the vector carries it across a length change; see ps_trim() for the whole contract. On a psw vector built from trimmed propensity scores, a subset drops it, while model.frame() re-attaches it to the shortened weights column of an outcome model fit on these weights.

is_unit_trimmed() therefore checks that the record covers the object it is given, and raises an error of class propensity_missing_meta_error when it does not, or when an object marked as trimmed carries no record at all, rather than name trimmed units at stale positions. Query the ps_trim object the record was written for instead.

That check counts observations, which a reordering does not change, so it does not catch one. A ps_trim reordered through vctrs rather than through [, by vctrs::vec_slice(x, 5:1) or dplyr::arrange(), keeps a record written for the old order, and a psw keeps one through any same-length operation, a reordering included. is_unit_trimmed() answers from those positions and names the wrong units. See ps_trim() and psw for the whole contract.

Usage

is_unit_trimmed(x)

Arguments

x

A ps_trim object created by ps_trim(), or a psw vector built from one.

Value

A logical vector the same length as x, where TRUE marks a trimmed unit.

See also

ps_trim() for trimming propensity scores, is_ps_trimmed() to test whether an object has been trimmed, ps_trim_meta() to retrieve full trimming metadata.

Examples

ps <- c(0.05, 0.3, 0.6, 0.95)
trimmed <- ps_trim(ps, method = "ps", lower = 0.1, upper = 0.9)

is_unit_trimmed(trimmed)
#> [1]  TRUE FALSE FALSE  TRUE

# Use to subset data to retained observations
kept <- !is_unit_trimmed(trimmed)
ps[kept]
#> [1] 0.3 0.6