Actual versus predicted treble string frequencies (Yeo–Johnson transformed, λ = 1.1301). Each point represents one instrument, coded by body material (symbol) and group membership (colour: bass-oriented = blue, treble-oriented = red). The shaded band represents the 95% bootstrap confidence interval around the regression line. Predictions show closest alignment for typical treble-oriented samples (mulberry body, 0.25 mm soundboards; Sect. 3.2), with larger deviations for transitional Sample 16, which combines treble-range materials with bass-like modal properties. Model was validated using leave-one-out prediction.