Evidence and Gap Maps

Evidence and gap maps are interactive, matrix-style evidence inventories that make it possible to see—at a glance—what interventions have been studied against which outcomes, where the evidence is strong, and where research is missing. Their value lies in helping funders and researchers avoid blind spots: instead of only asking what works, these maps show how much is known, what quality that knowledge has, and which gaps should drive future systematic reviews or new impact evaluations.

At the core is a structured “framework” that defines the map’s scope. Interventions typically form the rows and outcomes the columns, producing a grid of cells that can be populated with evidence items. Many versions also add filters—such as study design, population subgroup, region/country, and sometimes funding agency—so users can narrow the view to exactly the evidence they need. Campbell-style maps go further by attaching quality ratings and interactive features: bubble colors indicate evidence type and quality, bubble size reflects the volume of studies in each intervention–outcome cell, and clicking a bubble can open user-friendly summaries plus links back to the original sources.

The presentation contrasted evidence and gap maps with systematic reviews. Systematic reviews are narrower and aim to synthesize results to determine effectiveness; evidence and gap maps are broader and aim to catalog and organize what evidence exists. Both rely on rigorous, protocol-driven searching and screening, often with dual screening and conflict resolution. The key difference is what gets extracted and how: maps extract less outcome-level detail than systematic reviews because they are not designed to produce an effectiveness synthesis. That makes them quicker while still systematic.

A major theme was how evidence mapping evolved. Early work in the early 2000s used structured “evidence mapping” methods, with later breakthroughs bringing visualization and interactivity. By 2010, the International Initiative for Impact Evaluation (3ie) introduced evidence gap maps with interactive online versions. Over time, many organizations adapted the approach, leading to more than a dozen (and now many more) map-producing groups across sectors.

The talk also highlighted concrete examples of how Campbell has innovated beyond effectiveness. “Map of maps” aggregates completed evidence gap maps across international development topics, identifying priority areas for commissioning new maps. Mega-maps focus on broad domains like child welfare using systematic reviews only, then flagging where evidence is thin. In homelessness, Campbell developed an effectiveness map that critically appraised included studies—using tools such as an AMSTAR-based checklist for systematic reviews and a modified risk-of-bias tool for primary studies—then used the results to commission process-evaluation maps aimed at understanding barriers and facilitators. A country-focused Uganda map (2002–2018) illustrates the scale possible: hundreds of process, impact, and formative evaluations were captured, enabling country-level synthesis discussions.

Ultimately, evidence and gap maps function as building blocks in an “evidence architecture,” supporting knowledge brokering, evidence portals, and guideline development. They help identify where high-quality evidence exists, where absolute gaps mean no studies are available, and where “empty reviews” suggest systematic reviews may be premature. They also support research and funding priorities by reducing duplication—showing what others have already studied or are actively studying—so new work targets the most consequential missing evidence.