Methods and Approaches to Prove ROI of Marketing Consulting Engagements: An Empirical Attribution Protocol for Advisory Services

Marketing consulting is increasingly evaluated through quantified return on investment (ROI), yet advisory work often changes multiple levers at once - measurement, targeting, creative, automation, and governance - making attribution and incrementality difficult to defend under executive scrutiny. This article develops an attribution protocol tailored to consulting engagements that need auditability, causal reasoning, and practical feasibility under privacy constraints. Building on marketing productivity and metrics research (Rust et al., 2004; Seggie et al., 2007; O’Sullivan & Abela, 2007) and the attribution literature (Shao & Li, 2011; Dalessandro et al., 2012; Danaher & van Heerde, 2018; Berman, 2018), the protocol separates (a) incremental value created by the engagement from (b) channel-level credit assignment and (c) reporting conventions used by stakeholders. A simulation-based empirical demonstration shows how common rules-based approaches can misallocate uplift across channels and distort engagement-level ROI when the scope is defined around specific levers. When paid search and lifecycle improvements were the primary interventions, last-touch and time-decay approaches understated the uplift credited to those levers by roughly one third, shifting value to channels that were not improved. The proposed protocol integrates a pre-registered measurement plan, triangulation between quasi-experimental designs and attribution models, and a transparent financial bridge from marketing outcomes to contribution margin. Implications are offered for consultancies and clients seeking defensible ROI narratives in 2025–2026 conditions.