An evidence map helps readers see where research is concentrated and where a defined search found little or nothing. It is a map of the literature’s coverage. Before using its colours or counts to draw conclusions, find out what each cell represents and whether it describes study availability, study quality or actual results.
Read the axes as part of the research question
Campbell’s evidence-and-gap-map guidance describes a structured display of available evidence, commonly using intervention and outcome categories. Such maps usually show where evidence exists rather than what effect the evidence establishes.White et al. — Guidance for producing a Campbell evidence and gap map (opens in a new tab)
For a peptide topic, the rows might distinguish molecular forms and the columns might distinguish measured outcomes. A different map could organise models against measurement types. Neither structure is universally correct; it must fit the question stated in the title and methods.
Check whether a label such as cellular function covers one defined measure or several unlike measures. A broad label can hide useful differences even when every underlying study has been counted correctly.
Also check the date and eligibility rules. A map limited to a particular period, language or study design should not be presented as an unrestricted picture of all research.
Work through a small fictional map
| Form | Binding | Cell response |
|---|---|---|
| Parent | Studies A and B | Study B |
| Fragment | Study C | None found |
This original grid contains three distinct studies but four occupied study entries, because Study B measured two outcomes. Adding the entries would overstate the number of independent studies.
The fragment’s empty cell means that no eligible cell-response study was found for this fictional map. It does not mean the fragment has no cellular effect. It also does not show whether an excluded or undiscovered study exists.
Nothing in the grid tells us whether A, B or C found a larger response, a smaller response or an inconclusive result. Those findings would have to be inspected in the linked records or a separate synthesis.
Keep appraisal and direction separate from volume
If colour represents a quality assessment, read the assessment method and whether it applies to studies, reviews or specific outcomes. A green cell should not silently mean both many studies and a favourable effect.
In the fictional map, imagine Study A provides detailed methods while Study B leaves material identity unclear. Both still occupy their coverage cells, but a reader needs access to that difference before interpreting the parent-form evidence.
A filter can help focus the map on a model or outcome, but it also changes the visible evidence set. Record the selected filters when quoting a count, and reset them before describing the whole map.
When a map includes both reviews and primary studies, keep those categories visible. A review can summarise studies already shown elsewhere in the map; counting it as another independent experiment would be misleading.
Use gaps to ask a more precise next question
The empty fragment cell suggests a question about whether that combination has been investigated within the map’s scope. The next step is to examine the search boundary and relevant records, not to declare the biology settled.
The fuller parent row suggests there may be enough connected literature for a focused review. Whether those studies can be compared or pooled remains a separate decision about their actual methods and outcomes.
A useful map lets the reader move from a cell to the study identifiers and then to the original sources. This traceability makes it possible to investigate both apparent clusters and apparent gaps.
When summarising a map, describe the coverage pattern, its scope and the unresolved question. Reserve claims about effects for a synthesis that has examined the results themselves.
Sources and further detail
- White et al. — Guidance for producing a Campbell evidence and gap map (opens in a new tab)
2020 methods guidance read, including matrix purpose, counting across cells and map reporting. The parent/fragment grid and Studies A–C are original fictional examples; no real peptide evidence map or formal Campbell assessment is claimed.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.