DESI-18 Buddy Validation Study — Instrument & Prospectus (4-factor, v8)

DESI-18 is a 16-item dyadic interaction scale (4 subscales × 4 items) plus one attention check, partner-rated on a 7-point Likert scale, designed for single-session dyadic contexts. The 'DESI-18' name refers to the original 18-item pool from which the v6.0 16+1 final structure was derived. Includes scoring guide and the Buddy Validation Arm prospectus (v9). Data collection begins February 2027. Supported by the NIHR Three Schools Mental Health Programme Scholar Award (£10,900).