Extract questioned/reference/background samples from a list of populations from sources

The function splits a list of items (rows) into a sample of reference items, questioned items and background items.

make_idx_splits(
  sources,
  k_ref,
  k_quest,
  source_ref = NULL,
  source_quest = NULL,
  source_ref_allowed = NULL,
  source_quest_allowed = NULL,
  same_source = NULL,
  background = "outside",
  replace = TRUE,
  strict = FALSE
)

Arguments

sources	all class labels
k_ref	number of reference samples
k_quest	number of questioned samples
source_ref	the reference source (scalar; if `NULL`, a random source will be picked)
source_quest	the questioned source(s) (if `NULL`, anything but the reference source: behaviour overridden by `same_source`)
source_ref_allowed	allowed reference sources (if not `NULL` (default), reference source will be picked among these)
source_quest_allowed	allowed questioned sources (if not `NULL` (default), questioned source(s) will be picked among these)
same_source	if `source_quest` is `NULL` and `same_source` is `TRUE`, questioned source is the reference source; see details for conflict resolution
background	see details (default: `'outside'`)
replace	use sampling with replacement, else error
strict	fail at any incoherence between parameters instead of giving warnings or assuming (default: `FALSE`)

Value

list of indexes (idx_reference, idx_questioned, idx_background)

Details

Reference/questioned/background samples are always non-intersecting, even when the source is the same.

Sampling with replacement is used, if necessary and not forbidden. If it is used, a message appears.

It is always guaranteed that no sample appear more than once across reference/questioned/background items (but it can appear multiple times in a set if replace is TRUE).

Source sampling

Sampling happens in steps:

a reference source is picked
questioned sources are picked
items from reference sources are picked
items from questioned sources are picked
remaining items form the background set: apply background restrictions

By design, the package identifies sources which are allowed to be sampled from. By default, all available sources can appear in the reference or questioned samples.

This restriction can be modified using the parameters source_ref_allowed and source_quest_allowed. It is also forbidden to specify a source_ref or a source_quest which are not allowed.

The behaviour of same_source, when specified, is stronger, and source_quest_allowed is overridden.

If source_quest is NULL:

if same_source is NULL or FALSE, questioned items are sampled from all but the reference source (or source_quest_allowed).
if same_source is TRUE, questioned items are sampled from the reference source (source_ref_allowed is ignored).

If source_quest is not NULL:

same_source always has priority: if specified, source_quest will be ignored, the chosen reference source will be picked.
if source_quest conflicts with same_source, an error is raised.

Else, questioned items will be sampled from the questioned source(s), even if it contains the reference one.

Items will never be sampled once (unless replace is TRUE): they appear once in the reference/questioned/background items.

Background selection

If background is 'outside', the background dataset comprises all items who do not lie in any of the reference and questioned sets. It can contain items from reference and questioned sources.
If background is 'others', the background dataset comprises all items from the non-reference and non-questioned potential sources.
If background is 'unobserved', the background dataset comprises all items from the sources who do not appear in any of the reference and questioned sets.

By default, background is 'outside'.