Equate one clinical score scale to another (build a crosswalk)

Constructs a conversion from a "source" scale to a "target" scale using a linking sample measured on both. Equipercentile equating (the default) maps a source score to the target score with the same percentile rank, so it handles the non-linear, bounded relationship typical of ADL scales; linear equating matches only the mean and SD.

Usage

equateScores(
  from,
  to,
  method = c("equipercentile", "linear"),
  from_scores = sort(unique(from[is.finite(from)]))
)

Arguments

from

Numeric source-scale scores from the linking sample.

to

Numeric target-scale scores from the linking sample (equipercentile uses the two marginal distributions; linear uses their means and SDs).

method

"equipercentile" (default) or "linear".

from_scores

Source scores to tabulate (default: the sorted unique observed source scores).

Value

an object of class score_equating: a list with table (data.frame from, to), method, and the linking-sample size. Convert new values with applyEquating().

References

Kolen MJ, Brennan RL (2004) Test Equating, Scaling, and Linking; Houlden H et al. (2006) Clin Rehabil 20:153-159 (Barthel<->FIM).

See also

applyEquating()

Examples

set.seed(1)
ability <- rnorm(200)
barthel <- pmin(100, pmax(0, round((ability + 3) * 16 / 5) * 5))
fim <- pmin(126, pmax(18, round(18 + (ability + 3) * 18)))
eq <- equateScores(barthel, fim)          # Barthel -> FIM crosswalk
head(eq$table)
#>   from     to
#> 1   15 36.985
#> 2   20 42.000
#> 3   25 46.000
#> 4   30 52.000
#> 5   35 58.000
#> 6   40 62.000