literature-review

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---
name: "literature-review"
description: "Conducting systematic, scoping, and narrative literature reviews. Covers PRISMA/PRISMA-ScR protocols, search strategy (Boolean, MeSH), database selection (PubMed, Scopus, Web of Science, Embase), screening, data extraction, evidence synthesis (narrative, meta-analysis, thematic), and reporting. Use when planning or executing a formal literature review."
license: "CC-BY-4.0"
---

# Conducting a Literature Review

## Overview

A literature review systematically identifies, appraises, and synthesizes published evidence on a defined research question. The method ranges from informal narrative reviews to highly structured systematic reviews with meta-analysis. Choosing the correct review type, building a reproducible search strategy, and applying transparent inclusion/exclusion criteria are the foundational decisions that determine whether a review can be trusted and published in a high-impact journal. This guide covers the full workflow from question formulation to synthesis and reporting.

## Key Concepts

### 1. Review Type Taxonomy

| Review Type | Definition | When to Use | Time Required |
|-------------|-----------|-------------|--------------|
| **Narrative review** | Selective, expert-curated synthesis; no protocol; no PRISMA | Introducing a topic; describing mechanistic background | Days to weeks |
| **Scoping review** | Comprehensive mapping of evidence landscape; PRISMA-ScR; no quality appraisal | Understand what evidence exists before committing to systematic review | Weeks to months |
| **Systematic review** | Exhaustive search; predefined protocol (PROSPERO); quality appraisal; PRISMA | Answer a specific clinical/scientific question with highest rigor | Months to years |
| **Meta-analysis** | Systematic review + quantitative pooling of effect estimates | Quantify pooled effect size and heterogeneity across studies | Months to years |
| **Umbrella review** | Systematic review of existing systematic reviews | Synthesize evidence from multiple reviews on one topic | Months |
| **Rapid review** | Streamlined systematic review with time-limited methods | Time-sensitive policy or clinical decisions | Weeks |

**Peer reviewer expectations**: Journals in the biomedical domain expect systematic and scoping reviews to follow PRISMA or PRISMA-ScR reporting standards and to be pre-registered in PROSPERO (systematic reviews only). Narrative reviews are typically invited by editors rather than submitted unsolicited.

### 2. PICO / PICOS Framework for Question Formulation

Systematic reviews require a precisely defined research question. The PICO framework structures the question into searchable, operationalizable components:

| Component | Meaning | Example (for a clinical question) |
|-----------|---------|----------------------------------|
| **P** | Population | Adults with type 2 diabetes aged ≥ 40 |
| **I** | Intervention | SGLT2 inhibitors (empagliflozin, dapagliflozin) |
| **C** | Comparison | Placebo or standard care |
| **O** | Outcome | Cardiovascular mortality, HbA1c, eGFR decline |
| **S** | Study design (PICOS) | Randomized controlled trials only |

For basic science questions, adapt to PECO (Population, Exposure, Comparator, Outcome) or a custom framework. A well-formed PICO directly maps to search terms for each database.

### 3. Evidence Hierarchy

Different study designs provide different levels of certainty about causal effects. Standard hierarchy for intervention questions (highest to lowest):

```
Systematic reviews and meta-analyses of RCTs (highest certainty)
    ↓
Individual randomized controlled trials (RCTs)
    ↓
Non-randomized controlled trials / quasi-experiments
    ↓
Prospective cohort studies
    ↓
Retrospective cohort / case-control studies
    ↓
Cross-sectional studies
    ↓
Case series and case reports
    ↓
Expert opinion / narrative review / editorials (lowest certainty)
```

For diagnostic accuracy, prognosis, and etiology questions, the hierarchy diff