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Coerce a Triangle to a Link object

Source: R/link.R
as_link.Rd

Derive the development-link table from a Triangle and assign the Link S3 class so the associated summary.Link(), plot.Link(), fit_ata(), fit_intensity(), etc. methods dispatch on the result.

Unlike as_triangle() / as_calendar() / as_total() (which take raw experience data and validate/aggregate it), as_link() operates on a Triangle (already validated upstream) and reshapes it into link-pair rows. Each row corresponds to one development link (cohort, ata_from -> ata_to), the long-format intermediate underlying both the chain ladder (CL) and exposure-driven (ED) workflows.

Two modes are produced depending on exposure:

Single-variable mode (exposure = NULL)

The age-to-age factor is \(ata = value_{to} / value_{from}\), where \(value\) is the column named by loss.

Dual-variable mode (exposure supplied)

In addition to the loss-side ATA, the exposure-driven intensity \(g = \Delta loss / premium_{from}\) is computed and stored in the intensity column. The exposure measure is the denominator for loss ratio calculations; for long-term health insurance applications, risk premium is commonly used.

Usage

as_link(
  x,
  loss = "loss",
  exposure = NULL,
  weight = NULL,
  min_denom = 0,
  drop_invalid = FALSE
)

Arguments

x

A Triangle object.

loss

A single cumulative metric used as the link numerator. Must be one of "loss", "premium", or "ratio". Default "loss". For loss-side ATA this is the cumulative loss column, but any cumulative metric on the Triangle may be supplied.

exposure

Optional second cumulative metric, treated as the exposure base (denominator anchor) for the ED workflow. Must be one of "loss", "premium", "ratio", and must differ from loss. When NULL (default), only the single-variable columns are produced.

weight

Optional cumulative metric used as WLS weight in downstream summary / fit_ata calls. Must differ from loss. Cannot be combined with exposure (the dual workflow has its own anchor).

min_denom

Minimum denominator required to compute ata and intensity. If loss_from <= min_denom, ata becomes NA; if premium_from <= min_denom, intensity becomes NA. Default 0.

drop_invalid

Logical; if TRUE, rows with non-finite ata (single-var) or non-finite intensity (dual-var) are dropped. Default FALSE so the full link grid is preserved for diagnostics.

Value

A data.table of class "Link" with columns:

  • Always: [group], cohort, ata_from, ata_to, ata_link, loss_from, loss_to, loss_delta, ata.

  • If exposure is set: also premium_from, premium_to, premium_delta, intensity.

  • If weight is set: also weight.

The returned object carries attributes groups, cohort, dev, loss, premium (or NULL), weight (or NULL).

See also

as_triangle(), summary.Link(), plot.Link(), fit_ata(), fit_ed()

Examples

if (FALSE) { # \dontrun{
tri <- as_triangle(
  df,
  groups   = "coverage",
  cohort   = "uy_m",
  calendar = "cy_m",
  loss     = "incr_loss",
  premium  = "incr_premium"
)

# Single-variable: cumulative-loss link factors (ATA workflow)
link_loss <- as_link(tri, loss = "loss")

# Dual-variable: ED-ready link table (loss + premium)
link_ed <- as_link(tri, loss = "loss", exposure = "premium")
head(link_ed)
} # }